Absolutely disgracefull

Good Day. I was offered a discount on my account. Which i paid cash but unfortunately picked up a problem at one atm and then went to another atm. But the account was settled on the day it was due. Now i keep getting calls that i stil owe money. I have an sms whith the amount stated that needed to be paid. The more i tell them this the more they demand i pay the extra amount. I have proof of everythind and u email it and nothing happens. This is absolutely nonense. Why do they offer you a discount and then 2 months later demand more money. Everytime there is a different excuse. I paid my account and i want a settlement letter!!!!!! I will not fall for they bully tatics. I settled my part so do u part and sort this out