Many defense avenues.
First of all, you have a right to see ALL evidence/discovery prior to trial. Not sure of your state, but you can usually request that at pre-trial hearing. You'll be enticed with all sorts of really great plea bargains prior.
This is a tough battle to win, but keep in mind they have to prove beyond doubt that you actually committed theft. Whatever that definition may be in your state.
The "abandoned property" defense, with the "good faith" kicker is a good way to begin.
I know that in the Supreme Courts 6th district there are published court documents describing what a contract with a patron and a casino is. I'd have to re-look up the exact wording. In plane old english it states when someone puts money into a slot machine, they have entered into a contract with the casino. As soon as the patron wins a "round" or "hand", the casino gives up all rights to the money being played. The responsibility and rightful ownership then shifts to the patron. When the patron departs from the game, or machine, the contract is settled.
You can argue that translation within the publication as abandoned property. As the patron has declared no rights to that machine (including the ticket inside) by leaving. I don't know where your at, but the 6th District is in the Midwest/Michigan....etc...
Also, read the fine print on the back of these tickets. They are "bearer instruments"... Meaning there is NO identity attached. Whomever possesses such an instrument is the owner.
I lost in reading through a lot of garbage on this thread, did the original person sitting at the machine approach you for the ticket? Or was it the casino? Was there a complaint filed by the person at the machine?
Did you ever bother to look up casino policy? What does it say about "vacated machines with credits". That doesn't matter to a prosecution because State law will crush casino policy any day. But, if the casino policy says that it's not responsible for lost or even stolen tickets, it could back up your abandoned property defense based on the fact that they are admitting that it's the players responsibility to check tickets for accuracy and such, more importantly, being in possession of the damn thing. BECAUSE THEY ARE BEARER INSTRUMENTS and can be cashed/traded/negotiated/flushed down the ****ing toilet by whomever, without ever having to show identity!
Then there's the old, "well it wasn't yours in the first place"... Well a jury cannot use that in determining if the charges fit the crime your accused of.
In closing. Really? What dumb ass leaves their money in the machine at a friggin' casino? The real crime to me is more and more stupid people being born everyday! But you can't argue that in any court.

Also, in regards to evidence. Demand them to track the ticket, video, witness statements, etc... Then pour it on!!!
NOTE: I AM NOT AN ATTORNEY. I'M JUST A CITIZEN THAT LOVES OUR SYSTEM. IT'S 400 YEARS OLD AND VERY REFINED. I LOVE THIS **** AND ASPIRE TO BE A LAWYER ONE DAY. THIS WHOLE RANT IS MEANT TO NOURISH A DEBATE. ONE OF WHICH I WILL WIN. Thank you...