Here's the thing about Hossa.
If the Blackhawks trade him, what's stopping him from retiring? His salary will drop to 1M per after this season, which is nothing for a guy in his shoes. If Ottawa isn't competitive enough for his liking, or he was only staying in the league to add to what the Blackhawks as a group have accomplished and a trade insults him, he would retire and the Blackhawks would be hit with cap recapture. That would defeat the purpose of trading him away to get out of his 5M cap hit.
Now, in a fantasy land marshmellow clouds situation where Hossa wants to keep playing forever ala Jagr and will accept a trade from Chicago, I don't think it takes much to get him sent to Ottawa because if you look at Hossa's contract there aren't many non-cap teams in the league ready to be competitive who would take regressing a low salary high cap hit player with multiple years left like Hossa.
Ultimately, the only way CHI is probably getting out of the poison pill that is Hossa's contract will be via a legitimate unfortunate career ending injury that allows them to put him on LTIR. If they insult him by dumping him in a trade, he can just retire. If they buy him out, they still face cap recapture. If they keep him, there is a good chance he quickly regresses to a player not worth anywhere near 5M. (Look how quickly guys regress from year to year in their late 30s).