The team is already over the cap fully healthy, only way this works is with retention

No one is going to give up assets with long term for someone who's about to fall off a cliff compared to last season and a UFA following it.

This opens up a trade for Kerfoot

well your asset isn't signing with your team, so have fun with that

Done and done

I'm not a bug fan of Barbashev because I think he overachieved last year, but let's assume he didn't and he is a 20 goal scorer at a reasonable cap hit this year.

1) He is a UFA next year and the Leafs will almost certainly not be able to retain him.

2) The Leafs can't really afford that reasonable cap hit right now without retention.

3) Leafs don't need Barbashev. He'd be a nice to have (assuming we're getting the Barbashev of last season and not the one of the playoffs). We're not giving up Sandin for a nice to have.

Personally, I don't think the Leafs trade Sandin unless they're getting back another good young defenseman (think Chychrun, although I doubt that happens for different reasons). I imagine they would rather let him miss the year then give him up for something that they're not sold on.

Anyway, now that I've written this long explanation why the Leafs won't do it, I expect this exact trade to be announced within the hour.