Account messed up due to *********** staff

In january last year i went in to purchase a few items and was unaware that i had used up my credit available because I was not receiving my statement. The cashier had then failed to inform me that the store was offering a max credit limit for that day and it had to be paid back in so many days. Only when I went in to pay my account i queried why the balance was negative and it was then that the mistake was realised. they then informed me that i had to pay the entire amount of once. I explained to the manager that i could not do that and would not have purchased had i known this. But no one seem to care although it was not my fault. I have made several attempts to contact head office but no one would help. So we tried paying a little extra each month but kept getting into more of a mess. We have even paid R1700 last month but its still a mess. The current balance is R4083 with a payment of R2519 due. Thats more than half the balance due. prior to this the account was always up to date. This is all because a staff member was *********** and now my wife and i are suffering trying to get it on par but its becoming impossible. I cannot afford such an high amount. So what do i d