I had to get rid of a dog once, when my work/life schedule changed, and I wasn't able to properly care for it anymore. I took it to the ASPCA, who promised me they'd have no trouble re-homing it. But 20+ years later, it's still one of the things that really bothers me every time I remember it... and the damn dog's probably been dead for 10 years now!
I think there has to be something mentally defective about anyone who'd just dump a dog, without making sure it's able to go to a good home.
Our local ASPCA makes customers sign a contract stating they will take the dog back there, if they ever need to re-home it. Even if you already have a new owner agreeing to take it, they want to process them through their system, which I guess may just be one way they keep track of abuse and other issues. That's why I went back through them in the one case I had to do it, as that dog originally came to me through the ASPCA.