If someone wants to sign Butler to an offer sheet, and the Patriots decline to match the offer, it will cost the team a first round pick.
If Green Bay COULD sign Butler to a top-of-the-market deal (it would be very difficult for them to do so without making some significant restructures), I think the Patriots would be very happy getting Green Bay's first pick in compensation.
Considering we spent no draft capital to get him originally, and got two seasons of high-level play from him, I'd be pretty happy with it to.
Besides, who's to say Belichick couldn't flip that Packers pick into a 2nd and 3rd?
First round pick gets you the 5th option year. That's why I'd rather have it. A third round pick is pretty much a coinflip anyway.