NYR give up a promising LW who can also contribute now in exchange for much needed defense. Jacob Larsson probably plays in the AHL the rest of the year, but he can take an NHL job in 17/18 (with still 2 years of ELC, allowing the Rangers to pursue Shattenkirk in FA).
Cody Franson is not being utilized to his strengths in BUF. Sheltering him (offensive zone face-offs, PP time, playing with top 3 F lines), he can be a very impactful player.
Going into 17/18, the Rangers buyout Girardi, trade Staal, or lose Klein / Holden to Vegas in the expansion draft. They should lose at least 2 defenseman, so they can re-sign Franson for cheap if he is needed.