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I can show you how to create a credit memo and apply it to an invoice, quickjelly.

Here are the easy steps: 

1. Choose Customers, then select Receive Payments.
2. Choose your desired customer whom you wrote the refund check. You'll see the amount of the credit included in the Existing Credits field and the refund check at the bottom section of the window.
3. Select the Apply Existing Credits checkbox.
4. Enter the amount in the Payment column, if not already entered by QuickBooks, then select OK. 

You can check out the QuickBooks for Mac 2019 User's Guide for the detailed steps. Just press Command and F,  then enter Refund to locate the Creating a credit memo and refund section.

I'm just here if you need more help. Take care!