I dont think that Lehkonen brings anything more in a trade with the Rangers if salary\Cap retention is involved. At least, nothing impactful. It would just serve as a nice gesture because New York has 10M to play with under the cap this season and Lehkonen is on an expiring deal this year. It's next year, with two mega contracts kicking in that the cap space vanishes.
I believe that, as suggested by other posters, Lehkonen's value is greater in a package deal. With New York looking precisely for a bottom-6 player like Lehkonen to round out the roster, and with prior interest in Chiarot as well for a playoff run, maybe a package deal involving the two, or a greater deal, even, could be had.
New York wouldn't touch their NHL core in this deal, so it might prove interesting for them (keep in mind that there is no need for retention on expiring deals, but longer deals might seem more attractive with that consideration addressed):
To New York:
Chiarot: 3.5M
Lehkonen: 2.3M
Toffoli: 2.125M X 2 remaining seasons after this one
Rights to Harris
To Montreal:
Lundkvist
Kravtsov
1st round pick
Landing Lundkvist and Kravtsov would inject skill up front and on the back end with near NHL-ready (if not NHL-ready for a rebuilding team) prospects picked in 2018, almost four years ago.
Losing Toffoli is inevitable in two years' time with a rebuild on the horizon. In the meantime, unlike with New York, Toffoli won't be in a position to help Montreal reach the highest honours. Kravtsov, IMO, will inevitably provide a Toffoli production (or better) over the next ten years.
Drouin - Suzuki - Kravtsov
Hoffman - Dvorak - Gallagher
Pitlick - Poehling - Caufield
Byron - Evans - Armia
In short order, this year,s draft pick might infuse more talent in the Habs' lineup, either at C (to replace Dvorak as a 2nd line pivot, at D, to eventually replace Petry or on the wing, to play with Caufield).
Edmundson - Petry
Romanov - Savard
Guhle - Lundkvist
Price
Primeau