Very bad service

I cancelled my MTN contract and they confirmed that its cancelled,on the 08/10/2019 I receive a message that I owe 79.83 which I paid and when I ask why must I pay that money they said its because I paid less the previous month and when I ask for my initial contract the consultant are sending me from pillar to post,wht I dont understant is why was I paying more then my initial contract and no one can explain that to me