If you retain 50% on Rust with his deal I think the highest you are getting back is a 2nd.
I don't think you need to retain 50% on Rust to get a 2nd. Straight up, that's probably fair. If someone needs to send cap back, it might change things. The good news is the Minnesota is running pretty lean this season because the buyouts are tagging on $14mil. Next year, it drops to $1mil so they are are about to have some spending money. Fitting in Rust wouldn't be an issue especially if they trade Rossi who will need a new deal.
If there is any retention, it means the deal would have to be done in the off-season when out retention slots open back up. That's also when Rust loses his trade protection as well. I imagine Guerin will want a 1st rounder level value for Rossi so honestly, it wouldn't shock me to see something like:
Rossi+2rd 2025 for Rust+1st 2025 Top 10 protected*+Poulin+Kasper Bjorkqvist.