I wouldn't do it, personally. From the fact that it's not fair on Petra. She obviously has emotional issues caused by people before you took her on. By your own admission, she's been to several behaviourists who recommended euthanasia. Would you rather absolve your own guilt and feeling of failure by sending the poor dog to strangers she doesn't know, put her through a tough training regime she probably won't understand and be very frightened during and inevitably be found unsuitable - which you already know if you're honest with yourself - and be put to sleep in the end anyway. Or you can take your responsibiliti es head on and let her sleep in peace. You'll know where she is and the last thing she'll know is that you are with her and love her. It will break your heart to do it, I know that, but think of Petra rather than yourself.
Believe me, I do not say these things flippantly. It's hard enough putting an old, sick dog down to prevent further suffering, but a youg dog, it's simply awful.
Being cynical, this guy wants a cheap guard dog. I'm pretty sure he can buy a good, fully trained dog who is both stable and controllable. And how would you feel even if after all the training, she goes homewith this guy who is partially disabled and she snaps and attacks him like she has you? He can't get out of the way - she could potentially kill him and you could have a huge law suit in your lap, written contract or not. Even if she's kept outside as a guard dog, how on earth is she going to be with people if she gets absolutely no contact with them apart from being fed? And that's no life for a dog anyway.
edit: And can your daughter get her there safely without having to let her out of her crate? How would you feel if she bit your daughter and you weren't there to intervene?
