Interesting. Do you propose a straight trade or an offer sheet? An offer sheet in that range ($5-6.7M) is worth a 1st, 2nd and 3rd. Sens are seen as a playoff team, and with Benn in the lineup (his playoff production is usually better than the regular season) that Sens' 1st round pick is a 20th+.
No way Dallas goes for it. They need a quick turnaround and not a 5-year-plan. They had the league's worst attendance last year and have taken steps to address that (Roy, Jagr, Rome, Whitney). If they lose Benn for three picks they take a huge step back.
So, trade, then. Changing that 1st for Greening probably wouldn't be enough. For a rebuilding team, sure, Greening had a great rookie season, he's got size, speed and a hard shot. Not a replacement for Benn, but his contract would leave them a lot more room for the future. But Dallas is in need for a quick turnaround. Even if the team, is sold and moved the value of the franchise with Benn vs. Greening in the lineup is higher, and probably by more than $6M.
Also Benn is a center (or used to be) and in Ottawa he'd be behind Spezza and Turris. Niewendyk has already come out and said that he sees Benn centering Jagr and Eriksonn. Would Benn want to play left-wing in Ottawa (behind Michalek and possibly Latendresse) or stay in Dallas and play center with Jagr and Eriksson?
So, to summarize, I'm not even sure if Benn would sign an offer sheet, and I'm pretty sure that Dallas would match it if he did. And as for a trade, I think it will take more than Greening as the centerpiece of the trade from Ottawa's side. Given Michalek's and Latendresse's injury history I'd love to have Benn in there -- especially in the playoffs -- but we still have Greening and Smith who can both play left-wing.