Outstanding amount

I had a study loan with NSFAS 2012 when I started working I contacted them so that I can pay off the outstanding amount in installments. I started paying and also requested the statement of account so that I can settle the outstanding balance. There was no response,after two moths they send me an statement and the interest was more than R500-00, I am refusing to pay any interest that was accumulated with in the period .I called the call center and there was no assistance.