Prepayment of $4000 was applied to an invoice with $4800 leaving a balance of N800. How do we clear

Prepayment of $4000 was applied to an invoice with $4800, leaving a balance of $800 in the invoice . This invoice was paid in August 2014 but the balance of $800 is still showing in the invoice.

How can we clear the $800 from the invoice, you advise will be appreciated?