missing payment/instalment

I have an account with price and pride, and my last instalment was for R844.00. I paid the full amount and was given a receipt as per every transaction, this was on the 6th of October 2015, a few days later I received a call and was informed that i have not paid, I thought it has not reflected yet. A few weeks later, another call came through and to my surprise the same payment was requested but I know I finished paying the account and a month to date I am still dealing with the same problem being sent from pillar to post by Russells and Price and Pride. I cannot pay the arrears added to that account for their mistakes, i cannot understand how money can disappear within their system and no one can trace it ... i will not pay a cent more as I have fulfilled my contractual obligations and now they need to do what is right.