Poor accounting

Last month I have a mistake that they fixed on my statement that they overcharged me for a pair of shoes that I bought after they fixed it I got the statement that showed that my installment was 550 with nothing in arears end of April I paid them 100 short Now they want to come and tell me my arears is 280 and I must pay 830 I am not happy with this as it is clearly not right and on the email I receive they want to come tell me I have been paying short a couple of months although my arears showed that I was not in arears