I have negative amounts that keep showing up on my A/R aging report from previous bookkeepers.

A customer over paid on an invoice and it was marked to leave for use at a later time. Then a credit memo was created for the over payment and a check was cut. Now it keeps showing up as a negative on the A/R aging report and on the customer account. I am not sure how to clear this up. This was from a previous year so I am not sure how to go about taking care of this without it changing the financials for 2015. Can someone tell me how to clean this up?

Thank you