Neither did selling Davids to Juve for peanuts or choosing Dugarry and leaving Zidane to Juve, but those things happened.
I actually see Cassano pushing for a transfer out of Milan (the little playing time he got and mostly in meaningless matches is one of the reasons) and Milan willing to use him in some exchange deal. If other potential exchanges fail, then I won't be surprised if in the end Galliani goes for a Cassano-Amauri swap or maybe Amauri+money for Cassano.
Ah, again Alen with his unmistakable love for Milan and his 20/20 hindsight vision

Milan sold Davids because he was having character issues there and was not performing well enough mostly due to injuries. Both Dugary and Zidane were great at Bordeaux and Milan chose the player they needed more for their formation. Did they make the right decision in both cases, obviously not.
But how does that justify selling one of the most talented players in Serie A, who btw has proven to be very useful to Milan even after being out of football for 2 months, so could be great for Milan next year, for a player who is already 31 and has proven time and time again he cannot play on the big scene ...
I am not saying Cassano will stay at Milan no matter what, but if he were to leave, Milan would be looking for a player with a proven record in CL-level football, and will hardly just replace him with a Brazilian shemale, who can't get his act together even in Serie A unless he is playing for a midtable team.