Do the people proposing these 50% retained deals on Eric Karlsson even understand what that means? Karlsson is not a rental whose deal is up at the end of this season. He has four more years left on his deal after this one, during which time he will earn $38M dollars in actual salary plus whatever is left for this years. So, if you're asking San Jose to retain 50% of that, you're basically saying that you want them to give up Eric Karlsson and over $19M in real money plus take a four season cap hit of $5.75M a year. in exchange for what you're giving them.
If Muzzin is sent back in the return and he's able to play then SJ has to pay out another $4M and take a $5.625M for 1 year. If Muzzin is unable to play and his deal is fully insured then he won't cost them anything. I think the latter is more likely, so SJ taking him is probably just a favour to the Leafs.
So, basically, it comes down to what is $5.75M in dead cap for each of the next 4 years, $19M in real money, and Eric Karlsson worth?