Mavs get off Middleton and Martin, both players who arent really going to be beneficial for the future mavs roster, they recieve Lavine, a player who yes had a bad season, but could have a stronger final year season where the mavs could sign him to a smaller contract and trade for more assets, they also get the 2031 1st from sacramento which is the pick where the spurs have rights to swap so its more of a later 1st round pick that the kings could let go of, especially since they already have a 2031 minnesota 1st rnd pick. I think the mavs would do this trade kinda because of Masai having the track record to bring in expiring contracts for cheap and signing them into a team friendly contract, and the pick also follows that idea of how masai would find and pick up quality players deep in the 1st and 2nd round (take ideas like FVV who was undrafted, or OG who was a late 1st rounder, or even Siakam who was a post lottery pick, maybe more recent examples like Jamal shead in the 2nd round or Scottie barnes with the 4th overall during the Cade cunningham draft.)