stop payment on a personal check

What do you mean by "an optional contract?"  Does the contract provide that you can cancel it if you don't get the anticipated financing?  Does it provide that you have the right to cancel within a certain period of time?  If not, you are probably bound by the contract, although there are certain types of contracts which, by statute, you would have the right to cancel if you did so within a short period of time (often three days).  I don't know if there is any such statute covering the purchase of a car in California. Although you may be liable on the contract, you may not be liable for the full purchase price.  This depends on whether the dealership can sell the car, and whether it is what is known as a "lost volume seller."  If it sells the car (the dealership will try because it is under an obligation to try to reduce its damages), the amount you owe would be deducted from the amount it got from the subsequent purchaser (less some expenses, perhaps).  If, however, the dealership would have sold another car to the person who buys yours, had that person not buy yours, then the purchase price it receives will not be deducted from what you owe.  Probably the best example of this is a  hotel.  If you don't show up for a hotel reservation, and the hotel is able to fill your room, you will still be liable for the amount of the reservation, because the hotel would have been able to fit the new guest into another room if it hadn't given the guest the room you reserved.  If, however, the room  you reserved was the only vacant room in the hotel, you would only be liable for the difference between the amount you had promised to pay and the amount the new guest paid, because the hotel woudl not have been able to accomodate the guest and make that money had you filled your reservation.