Well i think flamingo took that DF for that reason alone, just to have something to hold over luffy's head. He's smart so he would know that he would clash with luffy at some point. Also, i think he is going to do more than just bait luffy with it. I think he will make luffy choose, tell him how he got it, and then give luffy a choice. He would have some one there about to eat it, some one luffy would not want to have it or maybe something like a rat or something stupid like that. Luffy turns or the DF gets eaten. I also thought about sanji, sanji is fast, more than fast. I think that maybe sanji speeds along to grab the DF and eats it to keep it from hurting luffy.
But that's probably just another crazy cross theory.