One way to do this deal is like this -
Spurs to Celtics - Malaki Branham and the 38th pick
Celtics to Spurs - Porzingis and the 28th pick
Spurs to Nets - Barnes, Keldon and the 14th pick
Nets to Spurs - Cam Johnson
This way, the Nets retain their picks and get the 14th one as well. They can try using the 8th and 14th to move up the draft or combine the 19th, 26th and 27th for future picks/ their preferred target.
The Spurs get Porzingis and can choose one among talent ranging from Rasheer Fleming to Ryan Kalkbrenner to Maxime Raynaud to Will Riley among others in the 27-30th pick range.
The Celtics get below the second apron by trading Porzingis and maybe even below the first apron ($2.78 million for its FRP + $30.73 million for Porzingis = $33.51 million - $4.9 million (for Branham) = $28.6 million.. that is just below the $29.9 million it needs to get below the first apron). It allows them to lose some salary for what is clearly going to a gap year with Tatum's injury and retool for later.
The Spurs get Porzingis and Cam Johnson to augment their frontcourt (Stretch wing/ SF/PF in the case of Johnson and shot blocking stretch 5/4 in Porzingis to back up Wemby). Branham is redundant for the Spurs with a strong guard rotation in Fox, Harper, Castle and Vassell.