What would you do?

So recently, the local news released a story.

A family signed a contract with the Humane Society to board their dog while they moved (they were in a situation where the dog could not stay with them temporarily as they moved from one house to another).

Unfortunately, a worker at the humane society did not double check the paperwork/follow protocol, and accidentally adopted the dog out to another family.

The mistake was caught within hours of the adoption, and they promptly called both families. Of course, the original family was upset and wanted to make sure their family pet would be returned.

The adoptive family, however, refused to bring the dog back. They feel like since they signed the adoption paperwork and paid the adoption fees, the dog is rightfully theirs. (The Humane Society did offer to fully refund them and/or apply them to another pet for adoption)