IF the invoice or bill you were presented with stated that for a party of your size, a 20% gratuity would be added, then they may keep the money; when that is the case, the law presumes you read and understood that term, and agreed to pay the base meal price, plus a 20% gratuity, plus whatever other or additional gratuity you voluntarily added.If the above is not the case, they may not do this: if it was an honest mistake, once you bring it to their attention, they should refund the money; if they refuse to correct the situation once they know they are not entitled to the money, this may be theft and you could report them to the police and/or sue for the money back.Alternatively, and a good thing to try first, contact your card issuer, explain the situation, and dispute the extra 20%; see if the credit card company can help you and get you the money back.