How do I issue a refund on an invoice that has already been paid in full? The invoice was paid in full November of 2019, refund check was issued in January of 2020.

Happy Friday, @doug32.

Glad to have you here in Community. 

The good news is, it's easy to record a refund in your QuickBooks Online. 

Let's get started:

1. First, click the + New button.
2. Choose Check or Expense.
3. Select the customer you want to refund from the Payee drop-down.
4. Then from the Payment account drop-down, click the bank account where you deposited the overpayment. 
5. On the first line of the Category column, choose an Accounts Receivable.
6. Enter how much you want to refund in the Amount field. 
7. After that, you can fill out the other fields as you see fit, then select Save and close. 

For more details on refunds in QBO, you can check out this article: https://quickbooks.intuit.com/learn-support/en-us/accounts-receivable/record-a-customer-refund/00/186177

That's all there is to it. If you have further questions or concerns, reach back out anytime. I'm always here to lend a helping hand. Have a great weekend!

issue the payment and use the sales income discount/refunds accounts

in QB you can not tie it back to the invoice, and it is a current activity so no back dating