fix final invoice and certificatre

certificates/views.py

@@ -78,7 +78,7 @@ def certificate_step(request, instance_id, step_id):
     if inv:
         if prev_si and not prev_si.status == 'approved':
             inv.calculate_totals()
-            if inv.remaining_amount != 0:
+            if inv.get_remaining_amount() != 0:
                 messages.error(request, 'مانده فاکتور باید صفر باشد')
                 return redirect('processes:request_list')