Thanks for dropping by in the Community, @toby1,
Are you referring to adding a credit memo to an unpaid invoice. If so, let me show you how.
1. Click the Plus (+) icon and select Credit Memo.
2. Choose the customer name.
3. Enter the Credit Memo Date.
4. Fill in the necessary information.
5. Click Save and close.
However, if you've already created a credit memo, you can proceed in applying it to the invoice.
1. Click Sales from the left pane.
2. Select Customers and click the customer’s name.
3. Locate the invoice you want to apply the credit to and click Receive Payment.
4. Enter the necessary information on the Receive Payment window.
5. In the Outstanding Transactions section, please make sure to select the correct invoice
6. Click Save and close.
To learn more when and how to give a credit memo to a customer, please read this article for more detailed steps: Enter and Apply a Credit Memo or Delayed Credit.
Should you need any further information, please do not hesitate to contact us. Have great day!