An invoice requires a payment to be marked as "Paid". If there is no payment or the payment is less than the value of the invoice, the invoice will be "Unpaid" or "Part paid".
Unpaid and Part paid invoices have outstanding amounts. This is the amount still to be paid (or has been over paid).
Outstanding amounts contribute to the Account balance calculation.
There are several ways to change or clear Outstanding amounts:
Pay the outstanding amount
In the invoice pop-up, Options > Take payment.
Take payment for the outstanding amount in the POS as usual.
Edit the invoice or payment
In the invoice pop-up, Options > Edit invoice.
Any item coloured blue can be edited by clicking on it. Change either the invoice or payment amounts so that they both match.
Void the invoice
In the invoice pop-up, Options > Void