When a customer paid more than what was owed and is requesting for a refund, you can create a Credit Memo and apply it to your invoice. Let me walk you through the steps.
To create a credit memo:
Click the Plus (+) icon and select Credit Memo.
Choose the customer name.
Enter the Credit Memo Date.
Fill in the necessary information.
Click Save and close.
To apply a credit memo to your invoice:
Select the + New button.
Select Receive Payment.
Fill in the necessary information such as the Customer, Date and Memo.
Under Credits, select Credit Memo.
Verify that the total to pay is correct after applying the Credit Memo.