As opposed to what alternative? When I bought my car, I wrote the check immediately after signing the contract (they surprisingly didn't need a cashier's check). Was he supposed to linger, or joy ride without paying? It makes perfect sense that he would go and get the check for the appropriate amount once finalizing the contract.
I would also say with great certainty that because of this: "Sawyer immediately left the dealership and returned with a cashier's check" that he KNEW EXACTLY what he was doing since when you go through the dealership financing
He very well might be an opportunist, but everything about the story shows that the dealership lied about some pretty big things, including telling the police that the car was stolen. What's one more thing? I don't know anyone that trusts a car dealership, who widely known for the bait and switch.