There is absolutely no problem with this. Mike Fisher is probably one of, if not the, classiest players on the sens. If he did not want to give up his #12 it just means that it is really important to him. It has been a tough year for the kid, he has been injured all season and he lost a very, very close friend in Roger Neilson, if he wants to keep the number leave it at that.
This is not a kid who just got here, he's been here for 5 seasons and is a major part of this team. Add to the fact that there is about 1000 (stretching the truth a little) #12 Fisher jerseys at the Corel Center on a nightly basis, the fans would be none to happy about that.