Since the application does not have this feature, you will need to make manual adjustment for this to work.
Please open the application and follow this path: Customers > Enter Customer Invoices / Credit Notes / Adjustments then enter a negative adjustment for the correct amounts and balance the journal using the account 6980 - W/O Specific Bad Debts. Next select Bank > Enter Receipts / Money In. From Customer Receipt enter the Customer Code and hit Tab, then a message will appear saying "There is unallocated cash/credit for this customer. Do you want to allocate one for these now?" Choose Yes. The list with transactions appears. Double-click the Adjustment that you just entered. At this point a message will appear saying "The receipt has been automatically allocated. Do you want to save this allocation now?" Click Yes and you're done.