FNB refuses to pay my refund for a settled loan

If i could give zero stars I would. FNB has been the worst at handling my concern. I settled a personal loan I had with them and they continued to debit my account. Requested for a refund and they refund me an amount that is not equal to the debited instalment. I then go to the branch and they assist me with calling their loans department (Vuyokazi or Vuyolwethu) who couldn't explain to me why they haven't fully refunded me. She then says she will transfer me to collections and she hung up. I call again and request the collections department numbers and speak toTlalane who put me on hold for 20 minutes busy checking my profile. When she returns she says, the reason why my refund is less is because they charged me interest. Mind you, the loan was settled same day I requested for a settlement quote.

I ask her to explain what she means, she asks me who gave me the settlement amount, I tell her that I requested for a letter and paid the amount as per settlement letter. She then says, there's an outstanding amount that they were supposed to add which was not added and they took it from the debited amount and refunded me the rest.

My question to her was, what is her understanding of a settlement, and she confirmed what I knew as well. So when asked why is the account still active after I've paid the settlement she had no answer.

I say to her, process the amount you took from me and she says if they do that then my loan account will be open again and I will owe them.

The way I am so upset, I wanted to drive from Rivonia to Bank City in Jhb and demand to see her and her superior. This is totally unacceptable!