Reversal

I have a loan account which is currently in arrears, so we had an arrangement that I will be paying full instalment amount and pay 500rand towards the arrears. Now what happend was come month end of November the whole arrea amount was taken off of 3650 instead of 500rand. So when I phoned in spoke to an agent they told me the reversal can't be done as they say I was suppose to arrange and in my mind the 500rand was an arrangement. Agent called the team leader Gloria who I pleaded with for an exception because I never thought such amount will taken off and its this time of the year. She said nope she will not reverse it I can take it up if I want. Im their client all I want is their understanding on the matter and I'm asking for an exception.