OpenCart Community

The case is as follows: At the beginning for one product there are 20 pcs. in stock. Client A ordered 1 pc., Client B ordered 3 pcs. and Client C ordered 10 pcs. After client C there are 6 pcs. from the product in stock as for this product was set Subtract Stock = Yes. I tried to change the payment method for order of client C in the admin panel but I can't save the changes because in the last step there is a message "Products marked with *** are not available in the desired quantity or not in stock!" for this product. These three orders are in pending status and the products in stock are subtracted. But because in the order there are 10 pcs. and in stock remained 6 pcs. there is an error message that prevent to save the changes in payment method. What can be done in this case? It makes sense to be checked in stock but if I try to add some product that are not available in the desired quantity or not in stock. It should not have an error message if the quantities do not change because they have been already subtracted.

Thanks in advance!