Edcon
Wrong billing

I went to one of their store to settle my acc, the gave me settlement amount and i settle the acc same day.
and they give me a receipt and promise that i will get a settlement letter in two weeks.
now to my surprise i receive a call from them saying that i have to pay them immediately an outstanding amount of 398 when i ask them how come they say a cashier made a mistake how can that be my problem.
i need an answer ASAP