Things are a little scattered because a lot of people are answering, so let's just break it down.
Did you call the restaurant to work this out with them?
Do you still have your copy of the credit slip?
Have you contacted your credit card company or bank regarding the alteration?
First of all, everyone so far has been correct with their advice. Here is how I recommend you handle this situation. Locate your copy of the slip so you have proof that it was altered. Next contact the restaurant, speak to the manager or owner, and tell them what happened. They may be willing to make it right. If they will not work with you tell them that you will be notifying your credit card company about the fraud. If after hearing this they still will not work with you then follow through with the threat.
Good luck and let us know how it went!