yea i had a prop stolen while the spectator was examinating it. He made me pay him 5 bucks to get it back
Next time tell him, "Ok you keep that, I want to show you another trick." Do anything you can think of, simple card trick whatever, that allows you to invade his personal space. He will be so concerned about you not getting the prop back that all of his focus is on that. While he is not paying attention to everything else, steal his wallet, car keys, sunglasses, watch, whatever you can see on him that is of value and as much as you can. Do the trick too, to keep suspicion down.
Then say ok how much is it for me to get my (whatever the prop is) back. He will say $5. You say, "would you trade for this wallet, these car keys, these sunglasses, this watch..." rob the idiot blind and make sure he knows that you will happily keep his stuff and replace the prop he stole on your own by selling his property, if he isn't willing to play ball.
What is he going to do? Tell someone on you? You are just returning the favor. A steal for a steal.