Broken agreement

Hi, I borrowed a loan from my pensions which was backed by fnb housing loan October 2018. So we made signed an agreement with fnb that I'm going to paid an installment of R2700 monthly which is being deducted in my salary advice but now fnb is keeping on deducting 3200 which I can't afford and I'm not coping as my net salary is very low I can't do anything. I writing this review so that fnb can start deducting the amount we agreed on in our agreement and refurd me all the deference starting from the day they broke our written agreement. Tha nk you