Loan payments

I took a loan with FNB a couple of years ago and I had some financial hiccups so that resulted in me paying them through their lawyers and administrators. 3 months back I made an arrangement with FNB to pay them directly through their legal department so I can speed up the process of paying off the money I owed them. I did make payments for 3 months and last month I was supposed to make my last payment and I did but FNB is denying me the letter that I have paid up all the amount I owed to them now they telling me they cannot see the payments in their system. I even got proof of payment from my bank (nedbank) and sent it to them to show that I have paid but still they are not attending to my query I call they dont answer they calls if they do the tell me the same thing of not seeing payment in the system I am tired to call FNB to give me my letter that I am done paying back the loan. Please help My details are Ref: *** Acc: *** Amount to pay: R325.00 FNB legal department: ***