Maybe you saved his life, maybe you sent him somewhere where he’ll run loose until he gets smacked by a car or attacked by coyotes or contracts a contagious disease like distemper and dies horribly. You have no idea because you don’t know the first thing about these people or the girl who picked them out. It happens. A LOT. And that’s assuming this girl even did send the dog to a nice farm family instead of to a puppy mill or a fighting facility, which also happens A LOT.
I say this as someone who has devoted her life to caring for and protecting animals and helping people give them the best lives possible–there are lives far worse than death, and there are deaths infinitely worse than an iv overdose of sedative.
As for the ethics question, I think what you did was highly unethical. The shelter, the people who spend their lives rehoming animals and evaluating potential new homes and actually knew this particular dog had already evaluated this chick and deemed her unsuitable to adopt him. You flat-out lied to them, signed a contract you had not the first intention of honoring, and got this dog into god only knows what kind of situation because you didn’t check out this girl or this other family before handing over a defenseless animal with an unknown temperament to their care. Evaluating a potential home isn’t just about kibble and shots and having a yard to play in and someone being home. It’s also about knowing a pet’s limitations and how that will factor in to a particular household–how it gets along with dogs, cats, children, strangers, whether it has medical issues and whether those are within an owner’s ability to deal with, how its personality fits with the general lifestyle of the home. You knew nothing about any of those things, so you were potentially setting up a really bad situation for everyone involved. That is so not cool.