Bad service

I have an account with truworts. My balance was more than 80000 as I bought a phone the I paid up my phone R5000 and before I paid I ask the cashier if they are gonna reduce my monthly installment she said yes. But after that I received the statement showing the same amount.. I called head office the guy I spoke to he said it is like that they can't change it. It looks like they don't give training to their cashiers