Bad customer service

My account was in arrears so I made arrangements with the debt collection agency to get a settlement letter and amount. Got the letter,paid the account 2years ago so now they have handed it to a different collection agency and they want me to pay again or I should send them proof of payment. So I went into the Edgars store in Vangate mall. They have informed me that i must go back to the store I made the payments at. I really don't know what to do anymore. Can somebody please help me?