Statement not making sense

I used my Foschini card to buy shoes for my brother in December at Sports scene , when I was trying to return them today at the same store I bought them and exchange them for a different kind of shoe they explained that they had to do a refund into the card which I expected. Suddenly they tell me that I have an outstanding on my account and I had to pay R 500 and something. This does not make sense because my monthly installment is R 735 on top of that I got my statement via Ema on the 21st of Jan which shows that I had to pay R 00 on the 1st of February I'm very annoyed and confused.