Can Collector Try to Collect From Me. I Lived Up to Agreement.

by Jody
(Atlanta, GA)

I was turned over to a collection agency for a lease I was on with an ex. The amount turned over was over seven thousand dollars, it was agreed upon by the landlord and the collection agency that I pay half and they would collect the other half from my ex since he was on the lease with me. I made monthly payments and paid my part in full and even have a "paid in full" final payment letter from the collection agency, as well as statements that specify I was only responsible for paying half the total amount. Well the collection agency has started harassing me for the remainder of the debt stating if I don't comply they will take legal action against me. How do I stop them from threatening me or calling me at all since I've kept up my agreement. Please advise.
Thank you in advance