This is another work around, but it presents a better product to deliver to the customer and for your AR department to work with. Read through to the end to understand the mechanics.

What is missing between an over payment and a credit memo is a clearing account

1. Create an invoice in the amount of the credit you would like to issue, and bill it to the same account from which you will enter the credit. Chose the day that you would like to 

2. Apply the overpayment(s) to the new invoice you just created

3. Create a credit memo for the same date and account as #1.

This is a work around, but it does allow you to issue a credit memo that can then be applied in the future. The overpayment creates a credit balance in AR, which is also what a credit memo does. Following the steps above will allow the credit to be transformed from an overpayment AR Credit Balance to a Credit memo.

The transaction journal looks like this, corresponding to the steps above

1. Invoice - Credit Sales, Debit AR

2. Payment - Existing Credit in AR canceled by New Debit AR created by the invoice in #1

3. Credit Memo - Debit Sales, Credit AR

Because the invoice and credit memo are on the same date, there is in essence no affect on sales. If you T chart this out and cross out the canceling/balancing transactions, there is no net change. But you have created a usable credit memo.