Transaction not processed

I took a car on the 4th of november and returned it on the 14th of november and everthing was fine, after a few days i received some refund. and this year i think in may i received an email from them saying i owe them money and that they dont have a transaction showing me paying the money. i have since closed my account with the bank i used the day i took the car. i asked them how can they refund me if my transaction has not went through. i have been trying to solve this issue with a guy from avis by the name of Marvin but he is useless as he does not answer my calls nor respond to my email, the only email he send me is that i have to pay the account. it has been month trying get an explanation as to why this happened but no one is helping, i cant pay twice.