Unfair service

I had opened a account with Nedbank and when my business was not doing well I decided to close the account. I waited until there was no money owing to the account and went and close the account. When i got to the bank they told me the account was owing but it was not reflecting on my banking app, the lady told me they could not close it because there was money owing, the problem is she also didn’t know where the amount was coming from because she even said she will ask the ask her manager to write off. Nedbank northga