Balance dropping, but installemnt stays the same

A few days before month end I had a live chat with the call center and explained to them that my balance is dropping but my installment is not. I also explained that the reason why i am doing this is because I am having financial difficulty. They agreed that they adjusted my installment, but nothing has changed. I paid the agreed amount that the agent told me to pay, but now my account tis showing R50 in arrears. I have never skipped a payment or paid less than what I was supposed to. Mr Price is ripping people off by not changing the installment when the balance drops and this is causing me much frustration tot he point where I want to close my 5 year account with them.