Nedbank Loan Overcharge

I hope that Nedbank can assist with this as I am trying to say this in the most decent manner. I had a loan in 2011 and due to financial difficulties could not keep up with payments. My loan was then handed over to a law firm and I have started repaying the loan. When I asked for a balance and statement of my loan, I got the shock of my life. My initial loan was R25859. The law firm now advised my balance is R27143.23. I then asked for the full statement and they sent me an incomplete statement. The balance after all the payment should beno more than R15361.05. I even showed my repayments which i am willing to do, however the law firm rejected this and expect me to pay double the loan amount back. This is completely incorrect and I trust a well known bank as Nedbank will be able to assist me in rectifying this matter.