Unfair deduction

I have a credit card with fnb that has been handed over to the lawyers. I made an arrangement with the lawyers to pay R100 to R250 a month.... However when ever there is an amount in my cheque account fnb will deduct it without my authorization. I went to the branch and they refused to reverse the deduction. Today I made a payment of R100 as per arrangements but fnb went and take R1000 on my account. I find it very disturbing because the reason I made arrangements is because I dont afgord to pay that amount.