I guess I finally convinced the rescue owner (I'll call her MV from now on) that talking on the phone with someone is different from meeting them in person. She sent an email to Bonnie, Clyde's new owner. She answered right away and this is the part of her reply that was forwarded to me:
"Clyde did great for his first night. He slept with my daughter, and seems to be attached to her. He did wag his tail for the first time when I got home from work in the morning. I am certain that we will bring him out of his shell, it will just take a little time.
Eighteen hours before he wagged his tail? He slept with the daughter who didn't want anything to do with him yesterday and now he "seems attached to her?" Then why didn't he wag for her?
He wagged for me as soon as we walked through the front door.

I know, it probably sounds like I'm nitpicking but I worked very hard at socializing him and introducing him to people and going places. He was fine! He never growled at anyone and wagged his tail for all but the carpet cleaner. He was fascinated with other dogs and friendly to them unless they were somehow intimidating.
Anyway, MV said she would keep a much closer eye on the situation than she does other "adoptees" but she said, "It's a done deal. I can't go to her and say, sorry you can't have the dog after all."
It looks as though he'll be theirs forever. I hate to think of him being so unhappy.