Do NOT sign it. In fact, write on the slip briefly about you do not agree on the amount billed because of the automatically added gratuity.

A local restaurant waitress did this similar thing to us last week.

The restaurant has those little machines on each table that the customer swipes the card, then the next screen is a 20% automatic gratuity added but the gratuity can be adjusted by sliding the bar left and right, BEFORE pressing the Pay this Amount button.

Well, this waitress first swiped the loyalty card, then my Gold card, then she just pressed a button, flipped the machine over, told me "you just need to sign it and you are all set." then she immediately walked away before I even had a chance to talk. I was like, What? I did not even know how much was the final amount to pay?!

The machine does not allow a cancel or back once at the final screen. So I used the Call Wait Staff button and the red light came on but it took almost 5 min before the waitress returned. I told her I wanted to know the final amount of the bill. She insisted the machine :"automatically did it."" BS. This is not the first time we dined there. I know exactly how the machine works - while 20% is automatically added, it definitely can be adjusted up or down just sliding the bar on the touch screen.

So I told her to get the manager. Took almost 10 min facing off before she went to get the manager. I was to the point that I would just get up to find the manager. Manager came and I explained to her what had happened. The manager opened the bottom of the machine, told me the transaction was now canceled and she would run me another payment slip to pay. On the manually produced slip, I crossed out the tip space, paid only the cost of the meal, wrote down the reasons (She had service issue too, gave us wrong side of the main course, but the manager fixed it when she came by to ask if everything was OK, earlier during the meal.)

My expectation from the manager's explanation was, I would see 2 authorizations on my Gold card but only one would eventually be billed. Well, it turned out there was only one authorization - the overcharged amount, showed on the card. There is no second authorization with the lower amount that I agreed to pay. I called the restaurant and the manager happened to pick up the phone and she knew it was me, by way of caller ID. She explained she issued a $5 refund to the charge, and I should see the credit in a couple days. It sounded that there was no way to modify the initial amount once the card was swiped thru the little machine, so a credit would be issued. At the end, what is finally posted on the Gold card, is the amount I agreed to pay, against the original authorization generated by the machine - I know that because I have set up an alert on the line item and eventually received an alert from AMEX, informing this pending charge was billed, but the amount now is the cost of the meal that of course I agreed to pay - the mechanism is different from what the manager told me, but with the same outcome.

In the past I often let such annoyance slide. Not any more. Now I feel I need to stand up against this sleazy scam some restaurants staff pull. If the customers continue to let them get away, they would just getting more blatantly scamming the customers.