I suppose the Japanese league could not make their players available for posting anymore. They could refuse to take Bobby Valentine back. I may be totally wrong but I really could see the Japanese league raise a fuss over this.
You're absolutely right.MLB doesn't want its teams interfering with guys under contract but this guy isn't under contract. I think MLB would balk if the guy wasn't 18 yet. That's the big double standard: MLB teams can go hoodwink Latin American players (other than PUerto Rico) without any agent representation as young as 16. MLB would have a cow if they did that in Japan or any country outside the developing world.