Credit limit

On the 24 July 2015 I went to Foschini at N1 City Goodwood and asked the cashier to check my total due for the month and she took my Foschini card and said R600.I paid the R600, I have now received my new statement showing total due R840 as the statement shows I failed to pay the full amount. I went back on the 25.08.2015 and made the payment of R840 and requested to take the interest off as it was not my fault but the sales lady. They contacted Foschini head office to correct the mistake but the guy Vincent said there was no interest charged and it was only R110 that was carried over to the new month that's why it shows R840 which I understood. I was also told by Vincent that I'm on 3 different interest plans which are 12 month clothing, 6 month clothing and 6 month cell plan. He advised me to make it to 12 month plan which will cost me less interest and pay 1 interest plan agreed and he changed it. On the 28.08.2015 i went to sport scene in Cape Town and wanted to purchase one top and was told I do not have credit. After I did a payment of R840 and on my receipt it shows you may still buy for R1223.00.