If Butler and the Saints can agree on a long-term extension, it's hard to imagine the Saints would go through the actual offer-sheet process as they aren't going to give up the No. 11 overall pick in the draft for Butler. But they would likely give up the 32nd overall pick they acquired from the Patriots in the Brandin Cooks trade -- or second- and third-round picks -- which New England would probably accept.
http://www.espn.com/blog/new-england-patriots/post/_/id/4801848/how-wes-welkers-07-patriots-arrival-could-foreshadow-malcolm-butlers-departure
http://www.espn.com/blog/new-england-patriots/post/_/id/4801848/how-wes-welkers-07-patriots-arrival-could-foreshadow-malcolm-butlers-departure