Hey there, @byomtov.

Thanks for reaching out to the Community for support. 

You have two different options when applying a credit memo to a specific invoice in QuickBooks for Mac. I've covered some steps below to help guide you through this process. 

1. Select the Company and Transaction Center and click on Credit Memos.
2. Choose a credit memo from the list on the left panel.
3. Make sure that it has the correct Customer or Customer job applied.
4. Click on the Use Credit to button drop-down.
5. Select Apply to invoice.
6. Choose which invoice you are going to apply your credit by ticking the box on the left side of the Date column.
7. Confirm the transaction by clicking Done.

An alternative way is to use the Receive Payments option. Here's how:

1. On the dashboard, click on the Receive Payments icon.
2. In the Customer Payment window, choose the customer on the Receive From box.
3. Tick the box on the specific invoice, then select the Discount & Credits button.
4. Check the credit that you are going to apply and click Done.
5. Hit Save and Close.

That should do it. Please don't hesitate to let me know if you have any questions or concerns about this process. 

Take care and have a great day ahead!