I want to try to give my dog back to his last owner
You don't need to do that. You can give him to someone else.
I don't think so. I can't trust anyone anymore so I'll return him to his original owner
The problem is, the owner may not want the dog back. They gave it to you for a reason.
It's him or no one else.
You're being unreasonable. People who give away their pets usually wouldn't want them back.