Unauthorized debit order/ double debit

Hi, My name is Tswarelo Ramjapedi. I have two personal loans with FNB, which are in arrears. I approached FNB last week to make an arrangement to pay two loan accounts, we agreed that they will R400 for one account and R800 for the second loan account, but FNB decided to debit a further R799 without me authorizing it. I tried calling FNB personal loans, all the agents I have spoken to keep transferring me to the other departments. All I want is a refund for the R799 that they debited without my consent. The loan account number is ***. I want my money back, Fnb can contact me on ***. My email address is ***