I think Boras is doing this kid a favor financially, but you may be right. The rep hit will be huge. Granted, Boras has never been one to shy away from being portrayed as the villain, so maybe he sends Matsuzaka back to Japan with everyone hating him, then come 2 yrs from now, Boras is again his client when he negotiates the mega deal.
Also, one may have to consider the rumor from a month ago, when there was speculation if Matsuzaka even wanted to play in Boston. Everyone knew he wanted to be a yankee, now he is being negotiated with by their enemy. It is interesting to watch this fold out.