Can a company hold me liable for an error on there part entering amount on card?

Roughly about a week ago I went and got some work done on my car. During the process of being cashed out the person cashing me out took my card, told me I owed 303.00 and processed the card. Well in doing such he made a mistake and instead of enter the $303.00 into the machine, he made the error of using the last 4 digits of my card as the total. $xx.xx. Over a week later I am getting a phone call from the business stating that "my card was not processed correctly" and that I needed to come in to fix it. As far as I am concerned and from what I have read, once I sign that credit card slip for $xx.xx amount, thats it. Thats what I agreed to pay, and thats what they had me pay. Am I wrong? Now that there attempting to get the rest of the money out of me, I dont know what to do. I know from a moral standpoint it would be the "right: thing to do, but this was there mistake and should be a lesson learned. So i guess my question is, 1. Can they charge more to my card with out me being there to sign something. 2. Can they legally force me to come in and pay the remaining balance. Like I said, once its signed and I walk out of the door and leave there property, thats that.