In the instances where your customer had made an overpayment, the overpaid amount can be forwarded to be paid on later invoices. Enter the payment as normal but post the payment with a positive amount in the difference field.
OASIS will warn you that you are attempting to post a payment with a difference. Select "Continue."
It will follow its standard 2 posting questions. The first will ask if you want to post payment, select "Post."
Then it will ask if you want to update accounting records, click "Post."
Then Save and Close