Unfairly handed over to attorneys by Miladys

I had an account with Miladys 2013. Outstanding balance was R500 and something rand. So I paid R70 per months. The last installment was R90 something. So I relocated to another town in 2014 and went to the shop assistents to pay my account. They told me that there was nothing outstanding on my account. So I left the matter there. Now after 4 years without any communication from Miladys, I all of a sudden receive a sms that my account was handed over to attorneys for collection for R415!!!! This is not fair. How can they just hand over an account of where the shop assistents at their shop told me there was nothing outstanding on the account???? So now the attorneys keep on phoning me for payment and everytime the amount differs. I've tried to phone Miladys head office to complain. They keep on referring me to the attorneys... why must I pay for someone else's mistake?