Customer balance Credit

Hello, i have a customer that should be a creditor. how can i specify this, is there any way?
Since this customer is used for pos and when i make a return on an invoice i get an error that says:
“Paid amount + Write off amount can not be greater than Grand Total”, I think this is because the balance of the customer can not be negative. and in order to submit the Return i should make a journal entry. So i need to make it directly without the need of a Journal entry each time. In other words The customer should be a creditor, can this be done? or there is other ideas?