Edited May 28, 2020 at 6:28 p.m.
From a Toronto perspective I'd swap you for positions of need.
A LHD for a RHD, Dermott for Montour or Miller. Dermott is a LHD who's shown hes capable of more than the 3rd pairing in Toronto (per just about any advanced stat you want to use, guys a darling in them) but won't get the opportunity behind Muzzin and Rielly. Montour/Miller are 2nd/3rd pairing RHD (if they even get the opportunity with Miller having been a healthy scratch for a good part of the season) who have had terrible seasons this year in Buffalo(again, their advanced stats this season are yikes). I'd be betting on them returning to form, or grow in Miller's case,, you'd be betting on Dermott continuing to grow. Were both hoping for 2nd pairing defensemen. I think I'd rather have Miller of the pair for his familiarity with the guys in charge but I think Buffalo might want to move Montour more.
I'd give you Kerfoot for Mittlestadt. Kerfoot is a considered a center for some reason but hes been better on the wing in both Colorado and Toronto, where he fit in just fine to the top 6 with JT and Nylander. Buffalo would have to move Skinner to the right side to make it work but with a pass first partner like Kerfoot I think they'd make it work. Kerfoot would also give you some flexibility in the center position where you are kind of shallow if any problems occur. Mittlestadt is frankly well on his way to bust and hasn't looked NHL ready at any point in 3 seasons of opportunities. He's spent most of this season in the AHL. Kerfoot is the more valuable of the pair at this point as Mittlestadt is pure gamble but I think I have a little bit of ground to make up in the defense.
All in all salaries come close to matching up so don't really need any shenanigans.