In VisionCore, Return Payment is locked from being edited. Normally when using the Return Payment it would be for returning the entire amount of the original payment. This should not change. If you want to change it then you would have to unpost the Original Payment (or Original Receivable) instead.
a. Returned Payment on Sales cannot be edited.
You have to unpost the Original Receivable if you want to change it.
b. Returned Payment on Purchases cannot be edited.
You have to unpost the Original Payable if you want to change it.