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