I recently purchased a used vehicle with a trade in. I live and purchased the vehicle in Illinois. We owned the trade in vehicle outright, and the point of the visit to the dealer was to get some cash back from the trade in, as we were ultimately trying to convert credit card debt into a lower interest rate car loan while getting a vehicle more appropriate for our family. We signed the contract for the car, that specified the sales tax and $17K that we were to get back in the form of a check. They told me that they would mail the check out next business day. A few days later, I get a call from the dealer saying that they did not collect enough sales tax (they miscalculated) and that they would need to deduct the difference from the $17K that they owe us. What are my legal options?