Charging Client for MTN's mishap

Cancelled our contract on 30.07.2017 and was advised the last payment needs to be paid at the end of August. Get an overdue message on 28 August, go to the store to check why I am getting the message and they advise there's a R260 that needs to be paid including the R798 for the contract as MTN did not cancel both numbers correctly on the contract.

The one number was still send the contracts airtime on the 21 August and now charging me for their mishap. Since my upgrade in 2015 I have never received the 25 sms every month, which results in MTN owing me 600 sms which amounts to R720 in sms. I don't want the money or the SMSes as it is too late and they never ever fixed this issue after many calls, but I refuse to pay R260 that is due to them. So they need to reverse the R260 and I'll just pay our last payment of R798, but MTN has not even budged to assist me with reversing the R260 and they just ignoring my requests with regards to the SMSes they owe me