How to ensure I'll get my dog back if she stays in a foster home for one month?

I have no choice but to leave my dog with a foster home for one month. She's a beautiful, well behaved dog that loves people. She's just a joy. I'm terrified that the foster home will want to keep her and refuse to return her. How can I ensure I get her back? Should I type up an agreement (I'll provide all of her things and she will be returned to me on August 1st) and sign it and have her caretaker sign it? Would that be enough for the police to retrieve my dog and return her to me if the caretaker refuses to return her?