Since Invoice is already Paid in full, therefore we cannot issue credit memo to the customer as there is nothing to apply. Both Invoice and Payment records are closed transactions.
Here are the steps on How to issue Credit Memo for the Invoice which is already Paid in Full:
1. Unapply the Payment to the invoice by navigating to Payment Record
2. Click Edit
3. Uncheck Apply checkbox (it will re-open both the Invoice and Payment records)
4. Navigate to the Invoice Record
5. Click the Credit button
6. Click on Save
The invoice now has a status Paid in Full by the Credit Memo issued to the customer.
7. In Addition, the payment made by the customer is also open and awaiting to be applied.
You may now issue refund to the customer by navigating to Transactions > Customer > Issue Customer Refund.
* In the Apply tab > set Apply checkbox = True for the Payment
* In the Refund Method tab > select Refund Method.
* Click Save