He was impounding it because he was called to do so by the property owner. He had every legal right to do that and did you a favor as he was not required by anything wait 30 minutes. I've done the same for people, because it saves them the storage fee that is a 12hr minimum (some places 24hr minimum) charge. Once a tow truck is hooked up to a car the tow truck driver is legally responsible for the vehicle until it is dropped off, if it is being impounded they can't legally just drop it back off, they will get fired and the tow company will get sued. That driver likely didn't pocket the cash, but a lot of driver are on commission (30-33% is normal) so he will get a chunk of it anyways.