There are usually a couple steps in restaurant approving credit card charges.

First they get an authorization for the amount of the bill and sometimes a 20 or 25% tip rounded up to the next dollar. A hold is placed on that amount.

They then put through the charge based exactly on the amount you write on the receipt. They could be batching that, where they process them and they don't get uploaded and settled for a day or two.

Are you sure it was the actual debit and not just the charge to hold it that went through? If it was from when they swipe the card, it will probably correct itself when the actual charge goes through.

If however it's the charge, you should bring it to either their attention or the credit card companies. If you are still in the area I would start with them, they probably just keyed it in wrong or read the receipt wrong.

It is however totally possible they are getting a couple bucks from everybody who hands them a credit card, hoping that most people won't notice or not want to be bothered with it, and could be making a ton of money.