Very bad service

My sister in law had a loan with FNB we received a settlement letter which include the following,Account number in the name according to our records the balance you owe in terms of the above-mentioned account as of the date of this letter is R4,805.31, this balance is currently due and payable. Your FNB Loan account *** proposed settlement amount is R 3844.25, proposed settlement amount needs to be paid within 30 days from the date of this letter. Should you not accept the proposed settlement amount and therefore not perform in terms of this letter by making payment of the settlement in full, your full outstanding balance will become due and payable.

An error was done and the R4805.31 were paid instead of the R3844.25.we got in contact with fnb and we were told that if that amount were paid its our loss and there is nothing that they can do ..Its january and we need our money back please