You could, but roster bonuses aren't fully guaranteed, which may cause issues with the Butler camp. And they can't be spread out over several years, though I think at a certain point, future ones can be converted.
So if Butler wants to get say $20M guaranteed his first year, a signing bonus would be the best option, especially since the Saints currently only have $8M of cap room to work with. The Saints could then guarantee money in future years if they want, but let's say we create a perfect deal for them, backloading the years because of their salary cap issues, then they bail on the deal at the last minute. Now we have a ****ty deal for us.
And that's really what I don't understand. Just because we CAN do this or CAN do that really misses the point. WHY would we do anything regarding negotiating a deal on our competitor's behalf?
Let's say the deal is for #32 and a 4th round pick. If we help negotiate a better deal, does that 4th become a 3rd? If we get the same pick regardless, who gives a ****? Just trade him, be done with it. Otherwise you're assuming a ton of headaches and risk for no real gain.
Maybe if the Saints are saying if you work out the deal for us, we'll upgrade the pick. But there's no indication that's in the cards, and quite frankly, I wouldn't be comfortable because you're still taking risk that they will bail on the deal last-minute. I don't think they would, but it's still risk that you don't need to do unless there's an actual financial gain somewhere, and I just don't see it.