You may run into a situation where an Invoice shows as "Open" but the Account Balance shows as $0.00:
Usually, this is the result of a Payment not being allocated to an Invoice. On the Account, these transactions would be highlighted in red:
To mark the Invoice as paid, open the Payment and allocate as needed:
When you click Save Payment, the page will reload and the Payment overlay will disappear. If you are on the Client Account, the red circle will disappear from the Payment and Invoice.