You're right; I forgot about Pryzbilla.
I doubt the Trail Blazers would part with Fernandez in such a trade, though. Webster, Pryzbilla, Cunningham and either Mills or cash might do it. Like you said, the Spurs could agree to an under the table buy out for Pryzbilla, so he could return to the Trial Blazers.
In the end, the Spurs would get presumably a better fit at SF than Jefferson, who's also younger and more affordable, plus a decent, inexpensive, young combo forward and potentially a lightning quick, young PG.