Is the thief's statement "my money is permissible," while knowing it is forbidden, considered a legitimization of sin?

This is not rendering the disobedience permissible; rather, it is a statement contrary to reality. Rendering something permissible (istihlal) is to say that theft is Islamically lawful. As for his claim of ownership over what is in his hand, this is not rendering the disobedience permissible, even if it is a lie.