Being a retail manager for many years, I'm inclined to believe that the OP is clearly wrong for violating a fairly obvious moral.
The way I see it, if I were to "mistakenly" leave a cash register drawer open, and some un-ethical individual was to take $35 out of that register, knowing it did not belong to them, they would most certainly be charged with theft. The fact that I made a mistake and left the drawer open has absolutely no grounds.
How is this different? You took money knowing it did not belong to you. This isn't a "finders-keepers" sort of situation. You knew that money wasn't yours, yet you took it anyway. The owner of that money has every right to make his claim to retrieve those funds.
How can anyone justify theft, no matter what the situation is?
The "customer is always right" philosophy has led to some pretty poor behavior and habits when it comes to self-entitlement. Don't get me wrong, I am fully aware that my paycheck relies entirely upon the customer. Does this give the customer the right to blatantly abuse unsophisticated systems? Absolutely not. No matter how you slice it.
What a shame ... AHS will likely not recover the loss, as I'm sure this guy feels entitled, in some crazy stretch of his imagination.