how do I link a refund receipt to a customer's account credit

Hi, Mark.

If a customer overpays, refund them from their credit and not with a separate Refund Receipt. This keeps their balance correct. 

To fix this, we'll delete the refund receipt you made, then we'll make a credit memo to record the credit and a refund expense to record the money you're refunding to the customer.

Finally, link the credit memo and the expense to show the credit has been refunded.

Here's how:

1. Select + New.
2. Choose Receive payment.
3. From the Customer ▼  dropdown, select the same customer.
4. Fill in the Payment method and Deposit to fields.
5. In the Outstanding Transaction section, select the checkbox for the Expense you created.
6. Select Save and close.

Note: Make sure the balance is $0, as the credit memo and expense will cancel each other out. 

Here's a link for more details: Refund your customer for a paid invoice in QuickBooks Online

Feel free to leave any additional questions in the comments if there's anything else you need further assistance with.