Biggest issue rehoming dogs like that, is that they become not just "a pet", but a member of the family.
Imagine as a kid you being disowned by your family and forced to live elsewhere... that's how the dog will feel.
Now they may eventually get used to their new family, but first the new family has to get past the massive hurdle of the dog thinking "Who are you, where is my family!"... which is generally met with the dogs only way to protect itself... aggression or aggressive signs.
So as a result the people get the dog, bring it home, it growls or bears teeth because it's scared and unsure of its new home, and before you know it, it's at the pound being surrended and put down, OR returned back to the seller.
BEST ADVICE I CAN GIVE YOUR MATE... Be part of the dogs rehoming.
Make sure he can go and take the dog with him to the new house a few times.
VISIT 1 - Bring him to new owners home and stay with him for a few hours in their surroundings, then bring him back home.
VISIT 2 - Bring him to new owners home and leave him for about 4-6 hours then pick him up and bring him back home.
VISIT 3 - Bring him to new owners home and leave him overnight (or two days) and see how he handles it.
If he handles overnight fine, then it's a pretty good indication you can visit less and less.
After that I'd say maybe visit once in the next week (if he handles overnight), and then maybe once the next month.
If you are gradual with working alongside him in his new home, theres a far less chance of him rejecting "new faces", and a far better chance him becomming part of their family and being accepted.
