Personal Loan Nightmare

Had a personal loan from FNB, i requested a settlement mount to pay off the loan. The consultant sent me the settlement amount which i then paid in full. A month later i see the loan account suddenly says -R938. Which means i still owe that amount. I phoned FNB and the consultant advised me that i paid too much on the settlement amount and then FNB credited my cheque account with the money i paid too much, FNB then made a mistake and paid too much into my cheque account which is why the loan account suddenly says -R938 so now i had to pay then more. I then paid that amount of R938 and then you guys promised that the account will be closed. This morning i see that you took interest of that amount after i was told you wont take interest because firstly i was given the wrong settlement amount, secondly you made the error to credit my account with too much, now i have to pay interest on your fault! as of today i am struggling to get a reversal on that interest amount! really the most ********** service i have ever received from a bank. Keep having to pay for your errors and not giving me my money back