Requesting a paid up letter!

What absolute nonsense. I have fully paid up the personal loan from that I had from FNB and I was told I will receive a paid-up letter after 30 days. 30 days later I called to request a paid-up letter and I was told I owe them R5000 +, because a reversal was made to my account. I have sent proof of payment and bank statement to show that I haven't received such amount! 2 weeks later, I still haven't received a paid-up letter and I am getting calls from debt collectors! I am so fustrasted with this!