How to get ‘Account Balance’ on invoice
You could use the customer statement feature. Granted, it’s not that great/user friendly, but it will show the open invoices as well as the open credits. If you’re wanting to apply those credits to the existing open invoices and have that show on the printed invoice form, you’ll need to modify the invoice form to include that information. Might be tricky/not possible to get it exactly how you want it, especially if you’re wanting it to show up as a line item, but at the very least you can definitely pull in the amount remaining (once you’ve applied the credits to the existing open invoices).