When an injury isn't one that will last the season, like Matsui, then I don't think there is an issue. If were to have definitely lasted the season, I would have offered the owner a redo on the trade. But to be honest, in my keeper leagues, the guys I play with would offer me the same. So it really boils down to what type of league you are in. In public leagues, I don't think you will see anyone doing this (or very,very few).