How to deal with incorrect credit card settlement

As per my agreement with their Collection Agent Deepak, It was agreed that I have to pay Rs 40,000 in "Cash" to settle my account in 2013, and the settlement letter will follow within a week after my payment. I learnt much later that I should have asked for the Settlement Letter prior to my Payment (Which I refer to this day). Since I do not have this much cash and was out of work, I got a loan from my mother to pay this amount. As it is a huge amount I decided to make online transfer from my ICICI bank to HDFC Bank account (Exhibit A - Payment Proof) where it is automatically debited. After which the Home Visit, Multiple calls/harassment stopped. I assumed every thing is closed and do not follow up on the Settlement Letter. After a while, I decided to get a call for the balance and all communications since then.. All of them Refused that I settled my account and continue billing me with Interest+Fees. Most of them are from their Call Center where they do not bothered to check my d