Break in payments for decemeber purchase and wrong Installment amount

I have a huge problem with the installment amount stated on my statement this month.

First of all there is a credit limit of R7700 on my account, i now see that the total owed is closer to R8700. Makro is now charging me an installment of nearly R1600! Please tell me how this is my fault? i went to purchase items Not knowing my limit, Makro's systems should have picked up that the limit was exceeded. I cannot afford to pay R1600 and this is not at any fault to me!

This needs to be rectified ASAP otherwise i will be blocking the debit order and paying exactly what i owe Makro according to my credit agreement. I have a credit agreement with Makro of R7700 not a cent more!

I also would like to know why i am being charged for my December installment as an email was sent by makro that if you purchase in December that i would get a payment break in Jan?