The OP has a good suggestion but it wont work with
how he wants it to.
At the deadline teams are looking to add players with low cap hits.
If we did give MSL 10M and retained 5M at the deadline the team
trading for him has to be able to absorb a cap hit of 2.5 to 3M.
When you pick up a player at the deadline, I think only about 1/4 or 1/5 of their salary counts towards the cap hit if you want to think of it that way because they're only there for a little bit of the season.
So you could have less then $2 mil in extra cap space at the start of the season... and by the deadline, you'll have enough room to add a $10 mil player with the Leafs retaining 50%... so in terms of cap space... the $10 mil player is basically like a $1 mil player in terms of cap hit if you pick them up at the deadline with 50% retained.
You can also do what Chicago did and have one of your players "injured" between the deadline and the playoffs... that fits everyone under the cap... then when the playoffs start, you bring your injured player back and you play way over the cap in the playoffs where cap hits don't matter anymore.