settlement letter

received a call from their collection agency blake and associates in October for a settlement amount which i agreed to , was told that i will be paying for three months which in October i couldn't and phoned back and was told that the settlement amount will stay the same and the first payment will be end of November. they debited my bank account for November, December and January, now that they have to send me a settlement letter i get a call stating that i was given an incorrect amount and need to pay a further R1000 odd rand..so they gave me an incorrect amount why must i suffer for that? i phone back to find out what happened apparently there is no trace on their system for the amount,,why would i be debited for 3 months and then the debit order stops? its not my mistake can i please get the settlement letter?