added test

app/controllers/admin/invoices_controller.rb

@@ -26,7 +26,7 @@ module Admin
 
       account_activity = AccountActivity.find_by(invoice_id: invoice_id)
       account_activity_dup = account_activity.dup
-      account_activity_dup.sum = -account_activity.sum
+      account_activity_dup.sum = -account_activity.sum.to_i
 
       if account_activity_dup.save and invoice.update(cancelled_at: Time.zone.today)
         flash[:notice] = t(:payment_was_cancelled)

config/routes.rb

@@ -232,7 +232,6 @@ Rails.application.routes.draw do
 
     resources :invoices, except: %i[edit update destroy] do
       collection do
-        # get ':id/cancel_paid', to: 'invoices#cancel_paid', as: 'get_cancel_paid'
         post ':id/cancel_paid', to: 'invoices#cancel_paid', as: 'cancel_paid'
       end
       resource :delivery, controller: 'invoices/delivery', only: %i[new create]

test/integration/admin_area/invoices_test.rb

@@ -4,6 +4,23 @@ class AdminAreaInvoicesIntegrationTest < ApplicationIntegrationTest
   setup do
     @invoice = invoices(:one)
     sign_in users(:admin)
+    
+    @account = accounts(:cash)
+    @registrar = registrars(:bestnames)
+  end
+  
+  def test_cancel_paid_invoice
+    @invoice.account_activity.update(sum: 10)
+    assert @invoice.paid?
+
+    assert_equal @registrar.balance, 100
+
+    assert_no_difference 'Invoice.count' do
+      assert_difference 'AccountActivity.count' do
+        post cancel_paid_admin_invoices_path(id: @invoice.id) + "?invoice_id=#{@invoice.id}"
+      end
+    end
+    assert_equal @registrar.balance, 90
   end
 
   def test_create_new_invoice