Fnb and debt collectors in cahoots with each other. Terrible bank

I had a personal loan with fnb which was handed over in 2018 to debt collectors who contacted me and I asked for the settlement amount, they gave it to me and I paid it, a few months later i was called again to say I did not pay the full amount and have an outstanding balance. I have proof of payments for both amounts but My credit is affected and I’m still owing fnb after paying what is due to them. I called fnb to find out they sent me around , till this day I have not been assisted.