Yes, I'd give it back. That's why I always count my change in the store or restaurant, and always check the bill. I don't want the place to get screwed over, but I don't want to get screwed over either. While I'm shopping a store, I always make note of how much stuff is and keep a running tally in my head. If it's similar at the end, then everything's good.

The same goes for missing money. I've found wallets and purses before, and I check the wallet for an ID. If I know the person or have seen them in the store, I'll give it back to them. If not, I return them to the Customer Service. But, as for money laying around, finders keepers.