Spurs part could be a small one. just take a look at Bulls financial situation:
The salary cap is projected at $56.1M. The max FA value for players like James, Bosh or Wade is $16,568,908 and Bulls want to add 2 of them.
Bulls' team salary:
Deng: $11,345,000
Rose: $5,546,160
Noah: $3,128,536
Johnson: $1,713,600
Gibson: $1,117,680
Max FA: $16,568,908
Max FA: $16,568,908
Roster Charge: $473,604
Roster Charge: $473,604
Roster Charge: $473,604
Roster Charge: $473,604
Roster Charge: $473,604
Roster Charge: $473,604
Total: $58,830,416
Bulls need to save $2.73M to have enough cap space for 2 max players.
Trading both Johnson and Gibson for nothing won't be enough because 2 additional roster charges will be created.
The Deng for Pryz + Bayless for Deng will save them $1.01M. They still will need to lower their payroll by $1.72M to have enough cap space.
Spurs will all their non guaranteed contract could be part of the equation. They can take 2 players among Gibson, Johnson and Bayless to put Bulls enough below the cap.