No do not sign. Never sign any blank documents. Never sign anything indicating they wish for you to sign something when clearly they lied about the condition of the motor vehicle. Inform them they will need to pay you your refund and you will sign a receipt indicating you received a full refund but that is all the statement should say. What they did could make them more liable for unfair and deceptive practices so you do need to stand your ground. If they still give you a hard time and refuse to turn over your money, contact the attorney general and the county or city prosecutor to file a criminal complaint.