The Texas Certificate of Title act prohibits sales such as you described. When he agreed to buy it, you should have transferred the title to him and placed a lien on the vehicle. Given that this started with a "deal," the police may or may not help you, and they and/or the DA may say that "it's a civil matter."
If the law enforcement folks won't help, you may need to sue him to get an injunction ordering him to give you the car. Unfortunately, injunctions are not cheap - you can't get one in small claims court. You may need a civil litigation lawyer to help you. Look for a local consumer law attorney on www.tbls.org.
Good luck. Next time you sell a car, do it right. If he has a wreck, commits a crime, or runs through toll booths or red light cameras, and the car is still titled and registered to you, you could face significant legal problems.
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