Actually, I dont think your scenario is possible at all.
If the suns were to do a draft day trade with Dallas, it would have to be after Dallas has chosen the pick. Draft day trades always deal with the "rights" to a player, not the pick itself. So if Dallas has already chosen the player, then we cannot send the pick to the Jazz (since it is now a player and not a pick). In other words, after the draft lottery, the Jazz will be sent the worst of the picks available. It doesn't happen on draft night.
However, about the salary cap issue. The way it would work is:
The mavericks trade the rights to their pick and another player for Jahadi White.
Draft picks have cap holds before they are signed. That way a team that is under the cap is forced to count their draft pick when figuring how much cash they have to go after FAs. So I would assume smallest contract the suns could take back would be: Jahadi White's salary minus the cap hold from the pick.
Assuming Dallas' first rounder is higher than 25, the cap hold would only be about 900k. So they would have to take back a contract in the 5 million range.
I like your waiver idea a lot better
