Shocking

My monthly istallment is R 357. Every single month I have been paying Russels R 400, which means my account is in advance. When I received my latest statement it stated that only R 84 is due. Due to constraints on my finances this month I only paid the R 84 and not the full installment as my account is already in advance. Today I received a call from Russels informing me I am in breach of contract because even if my account is in advance I need to stick to the installment. What???!! Which means Russels is very fast to accept my monthly payments that is far more than the agreed amount, yet if I only pay what is due ONE month, I get a phonecall. So what do you do if a person wants to make prepayments due to possible future financial constraints ? This is definitely against the CPA!!! Ridiculous!!!!