HTN Attorneys taking chances

I made an arrangement to settle my account with an amount of R3060.78. I made the payment of R3060.00, and forgot the R0.78. This payment was done on the 24th of October. HTN attorneys then calls to tell me that I have an outstanding balance of R300 and a couple of cents. Considering that it's not even month end why not ask that I settle the R0.78 how did we get to R300.00. I will not be paying HTN any more money and have referred the matter to my lawyers. What they're doing isn't within the law.