If a payable has a credit (for example, a refund from a vendor) and a debit that matches it, the net amount is zero.
In this case, the Pay Now button at the top of the screen is replaced by the Mark as Paid button.
When you click Mark as Paid, the status of the payable changes to Paid.
Note: Payables that net to zero can also be processed by the Make Payments function, but the method shown above is an easier way to handle them.