I don't see what your beef is... why do you need legal advice?
Let me get this straight.
1. You went to dinner
2. You ate ~$300 of food
3. They made a mistake, and initially charged you ~130
4. When they discovered the mistake, they correctly charged your account the ~$300 and refunded the ~$130
5. You now want to jew them for the ~$300 as well even though you ate it.
Getting the craps notwithstanding, do you see anything morally wrong about this on your part??
Put yourselves in their shoes - as a business, you sale an item to a customer. After running the card, and the customer leaves, you realise that instead of charging him $1000, you've stuffed up and charged him $100. Would you, as the proprietor, feel it is acceptable to amend the charges, or leave it? It's interesting when the shoe is on the other foot. It may sound harsh, but don't be a tight arse.