Pay me back

I rented a car on 26 november and paid a deposit to avis with my discovery card. I have been trying 5o get my deposit back but between avis saying discovery has the funds and discovery saying avis haven't given them anything I am lost as to what it is I need to do to get my funds back. I spoke to discovery today who now says there was never a payment. Thank god for the sms notification that I could use as proof. If it wasn't for that they would argue that there never was a payment. Even though there are previous requests from their own bankers to release the funds. Why would they do that if there was no payment. Just give me my money back ASAP.