i might be wrong but I'm pretty sure 10-5 rules only apply to trades. Since you can just let a player go to the team that claimed him without compensation I would think that the 10-5 rule doesn't apply in this case. There's not much you can get for the guy at this point anyway with his salary. I don't think cashman's the type to just let him go anyway, the pads were willing to take igawa off your hands and although I don't know the details it looks like cashman was asking for too much when he really is in no position to do so. I'm sure he would take the same attitude towards trading mussina.
of course if the red sox, mariners, tigers, indians, or angels claimed him I don't blame him for pulling him back