Do not use sale return lines for computation

issue7160
review41131002

sale.py

@@ -22,10 +22,9 @@ class Sale:
         amount = 0
         for line in self.lines or []:
             if (getattr(line, 'unit_price', None)
-                    and getattr(line, 'quantity', None)
+                    and (getattr(line, 'quantity') or 0) > 0
                     and not getattr(line, 'shipment_cost', None)):
-                amount += (line.unit_price
-                    * Decimal(str(line.quantity or 0)))
+                amount += line.unit_price * Decimal(str(line.quantity))
         context['amount'] = amount
         context['currency'] = self.currency.id
         return context