Being charged even though I made arrangements

I used to own a business Winistar and last year December we were liquidated, I owed nedbank R1800 which I have made an arrangement with their credit department that I will pay by month end Jan 2018. Yesterday they put more charges to the amount of R1000 due to interest even though I have made an arrangement. Now the amount is higher how will I pay that if I’m struggling to pay R1800. Please somebody assist me I need those charges reversed since I made an arrangement.