It is strongly recommended that you make a Backup of your invoice file before proceeding.
Go to the Payment tab in the program and find the customer that was overpaid. You will see a negative number in the Due column, indicating that an overpayment has been applied. To undo it so the customer will have a $0.00 balance, click in the Distribution column just to the right of the negative amount. Type a minus sign, and then the amount that was overpaid. Hit the Enter key on the keyboard, then click the Apply Payment button.
For example, if your customer owed $100.00 on an invoice, and you accidentally paid $1000.00, you will show an amount due of -$900.00. Click in the Distribution column and type -$900.00. Hit the "Enter" key on the keyboard, then click the "Apply Payment" button. The invoice will now show as paid the full amount and there will be no excess credit anymore.
Please note that any payment applied in this program will display in the "Payment History" dialogue. So in this case, both the incorrect $1000.00 payment will display there, as will the -$900.00 corrective payment. You can delete any unnecessary transaction listings when you are done on the payment history.