Being overcharged on loan fees that has been paid up

I had a 6 month loan with FNB which I took out in February. The last instalment was R879.32 and they debited my account with this amount on payday as per normal. I then noticed that I owed them R112.91. When I queried this I was told by one of their customer service agents that my instalment was R985.98. It is clear that what has happened here is that my last instalment was in fact R879.32. It clearrly stated this on my statement when I checked on my banking app. I queried this a few times and it seems that the customer service agents can't answer the question correctly. What has in fact happened here, is that I have been charged a monthly service fee of R69.00 and interest of R82.82 for August. In the beginning when I took the loan out I was given a quote for a total of R4 534.75. I received a message on my FNB app yesterday saying my loan account is in arrears. How can it be in arrears? There is no interest due for August as the loan is paid up!! FNB debited the amount that was due (Ie. R879.32) and now they want to add on these charges. I have tried emailing *** and the email comes back undelivered. I don't know how to get through to anyone there who can actually help me. Clearly this is just a ploy to make more money out of your customers. I do not owe this amount on the loan and would appreciate FNB reverse these charges. This is *****. I am horrified and disgusted at the no service I am receiving as one just cannot get through to anybody useful at FNB.