Disappointed with nedbank!!

I’m very disappointed with Nedbank, early August I took a loan with them. In September I decided to pay it up using the mobile banking app, using the balance that appeared on their app. Then beginning of October I realized That the 0.00 balance had interest how? How can you charge interest on a 0.00 balance? I don’t understand honestly. Today they debited R550! Why? I went to the KwaMnyandu branch as I wanted a clear explanation and a reversal of the debited amount. Nompilo checked my account then further called the loan department as she was also puzzled by this. She talked to the agent then transferred him to me so he called give me an explanation. At first he said I needed to call them for a paid up letter after transferring the money I further asked what is the point of having an app if you still need to use your airtime to call for a paid up letter, I then proceeded and asked how do they charge interest on a 0.00 balance? He asked me to wait as he had to consult to his manager, he came back saying his manager said the balance that I had paid was incorrect that’s why it fell on the following month of October. There were just too many stories that weren’t adding up. I’m just disappointed at their service. I regret switching to this bank and hasn’t even been a year with them. I want my money back. My reference number is ***7