diff --git a/src/app/Http/Controllers/API/V4/Admin/WalletsController.php b/src/app/Http/Controllers/API/V4/Admin/WalletsController.php
--- a/src/app/Http/Controllers/API/V4/Admin/WalletsController.php
+++ b/src/app/Http/Controllers/API/V4/Admin/WalletsController.php
@@ -7,6 +7,7 @@
 use App\Providers\PaymentProvider;
 use App\Wallet;
 use Illuminate\Http\Request;
+use Illuminate\Support\Facades\Validator;
 
 class WalletsController extends \App\Http\Controllers\API\V4\WalletsController
 {
@@ -45,6 +46,55 @@
         return $result;
     }
 
+    /**
+     * Award/penalize a wallet.
+     *
+     * @param \Illuminate\Http\Request $request The API request.
+     * @params string                  $id      Wallet identifier
+     *
+     * @return \Illuminate\Http\JsonResponse The response
+     */
+    public function oneOff(Request $request, $id)
+    {
+        $wallet = Wallet::find($id);
+
+        if (empty($wallet)) {
+            return $this->errorResponse(404);
+        }
+
+        // Check required fields
+        $v = Validator::make(
+            $request->all(),
+            [
+                'amount' => 'required|numeric',
+                'description' => 'required|string|max:1024',
+            ]
+        );
+
+        if ($v->fails()) {
+            return response()->json(['status' => 'error', 'errors' => $v->errors()], 422);
+        }
+
+        $amount = (int) ($request->amount * 100);
+        $type = $amount > 0 ? 'award' : 'penalty';
+
+        // TODO: create the transaction log entry
+
+        if ($type == 'penalty') {
+            $wallet->debit($amount * -1);
+        } else {
+            $wallet->credit($amount);
+        }
+
+        $response = [
+            'status' => 'success',
+            'message' => \trans("app.wallet-{$type}-success"),
+            'balance' => $wallet->balance
+        ];
+
+        return response()->json($response);
+    }
+
     /**
      * Update wallet data.
      *
diff --git a/src/resources/lang/en/app.php b/src/resources/lang/en/app.php
--- a/src/resources/lang/en/app.php
+++ b/src/resources/lang/en/app.php
@@ -38,5 +38,7 @@
     'search-foundxdomains' => ':x domains have been found.',
     'search-foundxusers' => ':x user accounts have been found.',
 
+    'wallet-award-success' => 'The bonus has been added to the wallet successfully.',
+    'wallet-penalty-success' => 'The penalty has been added to the wallet successfully.',
     'wallet-update-success' => 'User wallet updated successfully.',
 ];
diff --git a/src/resources/vue/Admin/User.vue b/src/resources/vue/Admin/User.vue
--- a/src/resources/vue/Admin/User.vue
+++ b/src/resources/vue/Admin/User.vue
@@ -136,6 +136,8 @@
                                     <span class="form-control-plaintext" v-html="wallet.providerLink"></span>
                                 </div>
                             </div>
+                            <button id="button-award" class="btn btn-success" type="button" @click="awardDialog">Award</button>
+                            <button id="button-penalty" class="btn btn-danger" type="button" @click="penalizeDialog">Penalize</button>
                         </form>
                     </div>
                 </div>
@@ -296,6 +298,42 @@
                 </div>
             </div>
         </div>
+
+        <div id="oneoff-dialog" class="modal" tabindex="-1" role="dialog">
+            <div class="modal-dialog" role="document">
+                <div class="modal-content">
+                    <div class="modal-header">
+                        <h5 class="modal-title">{{ oneoff_negative ? 'Add a penalty to the wallet' : 'Add a bonus to the wallet' }}</h5>
+                        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
+                            <span aria-hidden="true">&times;</span>
+                        </button>
+                    </div>
+                    <div class="modal-body">
+                        <form data-validation-prefix="oneoff_">
+                            <div class="form-group">
+                                <label for="oneoff_amount">Amount</label>
+                                <div class="input-group">
+                                    <input type="text" class="form-control" id="oneoff_amount" v-model="oneoff_amount" required>
+                                    <span class="input-group-append">
+                                        <span class="input-group-text">{{ oneoff_currency }}</span>
+                                    </span>
+                                </div>
+                            </div>
+                            <div class="form-group">
+                                <label for="oneoff_description">Description</label>
+                                <input class="form-control" id="oneoff_description" v-model="oneoff_description" required>
+                            </div>
+                        </form>
+                    </div>
+                    <div class="modal-footer">
+                        <button type="button" class="btn btn-secondary modal-cancel" data-dismiss="modal">Cancel</button>
+                        <button type="button" class="btn btn-primary modal-action" @click="submitOneOff()">
+                            <svg-icon icon="check"></svg-icon> Submit
+                        </button>
+                    </div>
+                </div>
+            </div>
+        </div>
     </div>
 </template>
 
@@ -310,6 +348,11 @@
         },
         data() {
             return {
+                oneoff_amount: '',
+                oneoff_currency: 'CHF',
+                oneoff_description: '',
+                oneoff_negative: false,
+                balance: 0,
                 discount: 0,
                 discount_description: '',
                 discounts: [],
@@ -412,6 +455,9 @@
             capitalize(str) {
                 return str.charAt(0).toUpperCase() + str.slice(1)
             },
+            awardDialog() {
+                this.oneOffDialog(false)
+            },
             discountEdit() {
                 $('#discount-dialog')
                     .on('shown.bs.modal', e => {
@@ -439,6 +485,16 @@
                     })
                     .modal()
             },
+            oneOffDialog(negative) {
+                this.oneoff_negative = negative
+                this.dialog = $('#oneoff-dialog').on('shown.bs.modal', event => {
+                    this.$root.clearFormValidation(event.target)
+                    $(event.target).find('#oneoff_amount').focus()
+                }).modal()
+            },
+            penalizeDialog() {
+                this.oneOffDialog(true)
+            },
             submitDiscount() {
                 $('#discount-dialog').modal('hide')
 
@@ -470,6 +526,34 @@
                             this.external_email = null // required because of Vue
                         }
                     })
+            },
+            submitOneOff() {
+                let wallet_id = this.user.wallets[0].id
+                let post = {
+                    amount: this.oneoff_amount,
+                    description: this.oneoff_description
+                }
+
+                if (this.oneoff_negative && /^\d+(\.?\d+)?$/.test(post.amount)) {
+                    post.amount *= -1
+                }
+
+                // TODO: We maybe should use system currency not wallet currency,
+                //       or have a selector so the operator does not have to calculate
+                //       exchange rates
+
+                this.$root.clearFormValidation(this.dialog)
+
+                axios.post('/api/v4/wallets/' + wallet_id + '/one-off', post)
+                    .then(response => {
+                        if (response.data.status == 'success') {
+                            this.dialog.modal('hide')
+                            this.$toast.success(response.data.message)
+                            this.balance = response.data.balance
+                            this.oneoff_amount = ''
+                            this.oneoff_description = ''
+                        }
+                    })
             }
         }
     }
diff --git a/src/routes/api.php b/src/routes/api.php
--- a/src/routes/api.php
+++ b/src/routes/api.php
@@ -101,6 +101,7 @@
         Route::apiResource('skus', API\V4\Admin\SkusController::class);
         Route::apiResource('users', API\V4\Admin\UsersController::class);
         Route::apiResource('wallets', API\V4\Admin\WalletsController::class);
+        Route::post('wallets/{id}/one-off', 'API\V4\Admin\WalletsController@oneOff');
         Route::apiResource('discounts', API\V4\Admin\DiscountsController::class);
     }
 );
diff --git a/src/tests/Browser/Admin/UserTest.php b/src/tests/Browser/Admin/UserTest.php
--- a/src/tests/Browser/Admin/UserTest.php
+++ b/src/tests/Browser/Admin/UserTest.php
@@ -465,4 +465,105 @@
                 });
         });
     }
+
+    /**
+     * Test awarding/penalizing a wallet
+     */
+    public function testBonusPenalty(): void
+    {
+        $this->browse(function (Browser $browser) {
+            $john = $this->getTestUser('john@kolab.org');
+
+            $browser->visit(new UserPage($john->id))
+                ->waitFor('@user-finances #button-award')
+                ->click('@user-finances #button-award')
+                // Test dialog content, and closing it with Cancel button
+                ->with(new Dialog('#oneoff-dialog'), function (Browser $browser) {
+                    $browser->assertSeeIn('@title', 'Add a bonus to the wallet')
+                        ->assertFocused('@body input#oneoff_amount')
+                        ->assertSeeIn('@body label[for="oneoff_amount"]', 'Amount')
+                        ->assertvalue('@body input#oneoff_amount', '')
+                        ->assertSeeIn('@body label[for="oneoff_description"]', 'Description')
+                        ->assertvalue('@body input#oneoff_description', '')
+                        ->assertSeeIn('@button-cancel', 'Cancel')
+                        ->assertSeeIn('@button-action', 'Submit')
+                        ->click('@button-cancel');
+                })
+                ->assertMissing('#oneoff-dialog');
+
+            // Test bonus
+            $browser->click('@user-finances #button-award')
+                ->with(new Dialog('#oneoff-dialog'), function (Browser $browser) {
+                    // Test input validation for a bonus
+                    $browser->type('@body #oneoff_amount', 'aaa')
+                        ->type('@body #oneoff_description', '')
+                        ->click('@button-action')
+                        ->assertToast(Toast::TYPE_ERROR, 'Form validation error')
+                        ->assertVisible('@body #oneoff_amount.is-invalid')
+                        ->assertVisible('@body #oneoff_description.is-invalid')
+                        ->assertSeeIn(
+                            '@body #oneoff_amount + span + .invalid-feedback',
+                            'The amount must be a number.'
+                        )
+                        ->assertSeeIn(
+                            '@body #oneoff_description + .invalid-feedback',
+                            'The description field is required.'
+                        );
+
+                    // Test adding a bonus
+                    $browser->type('@body #oneoff_amount', '12.34')
+                        ->type('@body #oneoff_description', 'Test bonus')
+                        ->click('@button-action')
+                        ->assertToast(Toast::TYPE_SUCCESS, 'The bonus has been added to the wallet successfully.');
+                })
+                ->assertMissing('#oneoff-dialog')
+                ->assertSeeIn('@user-finances .card-title span.text-success', '12,34 CHF');
+
+            $this->assertSame(1234, $john->wallets()->first()->balance);
+
+            // Test penalty
+            $browser->click('@user-finances #button-penalty')
+                // Test dialog content, and closing it with Cancel button
+                ->with(new Dialog('#oneoff-dialog'), function (Browser $browser) {
+                    $browser->assertSeeIn('@title', 'Add a penalty to the wallet')
+                        ->assertFocused('@body input#oneoff_amount')
+                        ->assertSeeIn('@body label[for="oneoff_amount"]', 'Amount')
+                        ->assertvalue('@body input#oneoff_amount', '')
+                        ->assertSeeIn('@body label[for="oneoff_description"]', 'Description')
+                        ->assertvalue('@body input#oneoff_description', '')
+                        ->assertSeeIn('@button-cancel', 'Cancel')
+                        ->assertSeeIn('@button-action', 'Submit')
+                        ->click('@button-cancel');
+                })
+                ->assertMissing('#oneoff-dialog')
+                ->click('@user-finances #button-penalty')
+                ->with(new Dialog('#oneoff-dialog'), function (Browser $browser) {
+                    // Test input validation for a penalty
+                    $browser->type('@body #oneoff_amount', '')
+                        ->type('@body #oneoff_description', '')
+                        ->click('@button-action')
+                        ->assertToast(Toast::TYPE_ERROR, 'Form validation error')
+                        ->assertVisible('@body #oneoff_amount.is-invalid')
+                        ->assertVisible('@body #oneoff_description.is-invalid')
+                        ->assertSeeIn(
+                            '@body #oneoff_amount + span + .invalid-feedback',
+                            'The amount field is required.'
+                        )
+                        ->assertSeeIn(
+                            '@body #oneoff_description + .invalid-feedback',
+                            'The description field is required.'
+                        );
+
+                    // Test adding a penalty
+                    $browser->type('@body #oneoff_amount', '12.35')
+                        ->type('@body #oneoff_description', 'Test penalty')
+                        ->click('@button-action')
+                        ->assertToast(Toast::TYPE_SUCCESS, 'The penalty has been added to the wallet successfully.');
+                })
+                ->assertMissing('#oneoff-dialog')
+                ->assertSeeIn('@user-finances .card-title span.text-danger', '-0,01 CHF');
+
+            $this->assertSame(-1, $john->wallets()->first()->balance);
+        });
+    }
 }
diff --git a/src/tests/Feature/Controller/Admin/WalletsTest.php b/src/tests/Feature/Controller/Admin/WalletsTest.php
--- a/src/tests/Feature/Controller/Admin/WalletsTest.php
+++ b/src/tests/Feature/Controller/Admin/WalletsTest.php
@@ -64,6 +64,56 @@
         $this->assertTrue(!empty($json['mandate']));
     }
 
+    /**
+     * Test awarding/penalizing a wallet (POST /api/v4/wallets/:id/one-off)
+     */
+    public function testOneOff(): void
+    {
+        $user = $this->getTestUser('john@kolab.org');
+        $admin = $this->getTestUser('jeroen@jeroen.jeroen');
+        $wallet = $user->wallets()->first();
+        $balance = $wallet->balance;
+
+        // Non-admin user
+        $response = $this->actingAs($user)->post("api/v4/wallets/{$wallet->id}/one-off", []);
+        $response->assertStatus(403);
+
+        // Admin user - invalid input
+        $post = ['amount' => 'aaaa'];
+        $response = $this->actingAs($admin)->post("api/v4/wallets/{$wallet->id}/one-off", $post);
+        $response->assertStatus(422);
+
+        $json = $response->json();
+
+        $this->assertSame('error', $json['status']);
+        $this->assertSame('The amount must be a number.', $json['errors']['amount'][0]);
+        $this->assertSame('The description field is required.', $json['errors']['description'][0]);
+        $this->assertCount(2, $json);
+        $this->assertCount(2, $json['errors']);
+
+        // Admin user - a valid bonus
+        $post = ['amount' => '50', 'description' => 'A bonus'];
+        $response = $this->actingAs($admin)->post("api/v4/wallets/{$wallet->id}/one-off", $post);
+        $response->assertStatus(200);
+
+        $json = $response->json();
+
+        $this->assertSame('success', $json['status']);
+        $this->assertSame('The bonus has been added to the wallet successfully.', $json['message']);
+        $this->assertSame($balance += 5000, $json['balance']);
+
+        // Admin user - a valid penalty
+        $post = ['amount' => '-40', 'description' => 'A penalty'];
+        $response = $this->actingAs($admin)->post("api/v4/wallets/{$wallet->id}/one-off", $post);
+        $response->assertStatus(200);
+
+        $json = $response->json();
+
+        $this->assertSame('success', $json['status']);
+        $this->assertSame('The penalty has been added to the wallet successfully.', $json['message']);
+        $this->assertSame($balance -= 4000, $json['balance']);
+    }
+
     /**
      * Test updating a wallet (PUT /api/v4/wallets/:id)
      */