You are responsible to pay the lawful amount of tax you should have paid. That amount is set by law, not by the dealership; it's not up to the dealership to determine your sales tax. Furthermore, the law recognizes that people sometimes make errors on contracts; if an error is typographic error, a mathematical error, or the like, a court would still enforce the contract and simply "reform" (correct) it to reflect what it should have been. For both reasons, you have to pay an difference between the sales tax charged you and the sales tax you should have paid.