I doubt Diaw opt out and I doubt you could trade him from nothing. Teams with cap space won't really be interested in a 31 years old Diaw averaging 5.8 ppg and 3.4 rpg with back issues and with a questionable motivation/work ethic.
Unless he had a great playoffs run, the only way I can see Diaw opting out is if Spurs do like with the other Jefferson and promise him a long term contract if he does it. A $10-12M/3 years contract sounds like a reasonable deal for both sides.
And at this stage at the playoffs, the question isn't really if Spurs would be fine with having Diaw on a $4.7M expiring contract, it's if they would be fine with paying Splitter what he will get in the FA market. Do you really want to give him something like $32M/4 years after what he is doing in these playoffs?