Do you know this--for a fact?--or did someone tell you this?
Because I'm thinking you are out of luck here. I will look it up in the am, but I don't think you can force mom to repay this 800$.
That appears to be the case.
First citation is an attorney who has offices in both NY and NJ. They didn't specify which one they were talking about:
Over-Payment of Child Support Can Only Be Re-Couped Against "Add-Ons" : New York Divorce Report : Daniel E. Clement: New Jersey & NY Lawyer & Attorney for Family Law & Pre-Nuptials
"But what happens if the parent paying support is current in making support payments under an existing order and is granted a downward modification? There would have been an over-payment of support. Could the parent paying support further reduce the child support payment to recoup the overpayment? The Appellate Division, First Department answered that question with a unanimous “No.”"
This appears to be the case in most other states, as well. Once paid, CS is very difficult to recover.
NJ's site says that ending the order does not resolve overpayment issues:
http://tinyurl.com/3plb443
That clearly says that if OP want to recover his money, he needs to file a separate action in court to recover. First, this might or might not be successful, and even if it was, it would take a great deal of time and possibly money to pursue it. It's almost certainly not worth it for the amount involved.
I hope that others reading this learn a lesson - people should file to have CS terminated before the qualifying event. You know when the child will turn 18 or graduate from high school, so get your paperwork in early.