Do you still have your old car? If yes, you likely can tell the dealership that you have changed your mind about going through with the purchase.
You may possibly have an enforceable contract to buy the new vehicle. However, given that only a few days have passed during which the values of the two vehicles likely have not changed and the dealership has not suffered any measurable damages, it is unlikely that the dealership would go through the trouble of suing you.
The seat belt has to be repaired whether the vehicle is sold to you or to a different buyer. So, repairing the seat belt is not a damage to the dealership.
If the only reason why you are hesitant to buy is possible mechanical troubles with the new vehicle, you can have your own mechanic inspect the vehicle and let you know whether major repairs will soon be needed.
If you do not have your old vehicle in your possession, the dealership may be telling you that the vehicle has already been sold.
If you have been a repeat customer at the dealership, the dealership likely would work with you to keep you as a customer.
You can test drive it and state it did not pass the test and you are back where you should be, if that is what you want.
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