I over paid a collection agency. They were receiving my payments by pre-paid checks, when debt was considered paid I realized I had over paid. It isn't a lot of money but they
Wouldn't stand for it so neither am I. The agent in charge of my file & the office manger refuse to reply.
What can I do?

Before considering anything just write them a letter, address it to whoever is in charge, try and get a managers name and send proof of all the payments along with their invoice. Make sure and send it registered so you have the proof if you need it later.