We spent a lot of time counting the guy's change today. He did have enough to get a new Lego set. But I guess we didn't explain that you do have to give up the money to get the goods. When it came time to pay up, he found it to be a surprise.
He thought that money worked like a reward system, rather than as a medium of exchange. He thought that the way that buying things worked was that you accumulated money and then received prizes based on the amount of money you accumulated, but you never ever actually lost money.
(This is how our sticker system works. Every n stickers, you get something, but the stickers are never taken away.)