It's possible. But as with most such scenarios, to me that's a wasted worry, because the chances that Fred would ever have become that player with the Bruins are very low. If he does go elsewhere and excel, I would simply say well done to him. Not really a Boston issue anyway - been a long time since any player we've cast off has really stepped up with another team.
That said, of course the letting him walk for free part would be the irritant. If Sweeney doesn't see him in the Bruins' long term plans or feels he's not close on negotiating an agreeable deal for both parties, then don't hesitate, trade him. Some return value is better than none.