Settlement of an account

Hey I have an account with MTN and i couldn't pay my account for months until they reach out to me regarding my outstanding balance of R23428.00 and then we made payment arrangement to settle that amount in 4months and in 4months time i manage to settle that amount. now they are coming again saying that i owe them R2760.00 saying its for collections commission, how so when we had agreed with the first amount not this one. please assist.