Good to see you, global2000.

In QuickBooks Desktop, the customized columns are non-calculating fields. We can only manually enter information in it. 

What I can suggest is to create a new item for retention. Then, add it when you create a new invoice. These are the steps in creating a retention item:

1. Go to Lists and select Item List.
2. Press Ctrl + N on your keyboard. Select either Service or Non-inventory as the Type.
3. Under Account drop-down, select an expense account
4. Enter all details needed, then hit OK to save it.

Create a new invoice as usual, then add the retention item. 

If you're using QuickBooks Online, customized columns are still non-calculating fields. You'll want to do the same workaround. 

1. Select Sales tab and click Products and Services.
2. Click New. Fill out the information required.
3. Hit Save and close.

Here's a guide in recording invoice payments in QuickBooks Online. That article will show the steps on how to properly enter customer payments.

Keep in touch if you need more help. The Community Team is always here for you.