if the 'rescuer' was well aware of the contract, I would have 'thought' the ethical thing to do would have been to contact you
Since the rescuer won't give you the adopters information, I would explain that you want to make sure the dog is in a good home vs enforcing the contract, and ask the rescuer to have the adopter contact you...That way the adopter has the ball in their court, if they don't, well I"m not sure how you could track them down
I also commend you for being dedicated in finding out if he has gone to a good home