He'd need to approve any deal because of his full no-trade clause.
Meanwhile, Marlins MLB.com reporter Joe Frisaro notes that the deal is not imminent and that the Marlins are still talking with other clubs.
A trade framework was a logical precondition for the Marlins to grant the Giants and Cardinals permission to speak with Stanton. The Los Angeles Dodgers have also been linked with interest in Stanton.
SiriusXM radio host Craig Mish first reported news of the meeting between reps for Stanton and the Giants. Per MLBPipeline.com, those are the Giants' second-, third- and fourth-rated prospects, respectively.
It's unclear what Miami would target in a trade with the Cardinals, and the deal may prove to be a nonstarter. But the Dodgers have luxury-tax issues and have not seriously pursued Stanton - at least not yet.
Miami is looking to slash payroll and Stanton is owed $295 million over the next 10 seasons as part of his 13-year, $325 million contract.
That's another factor, the state of the Giants and whether they can convince Stanton the team has a bright future despite the team-wide struggles starting after the 2016 All-Star break and a core of players advancing in age - the Dodgers' core is far younger. Stanton could ask that the opt-out be moved to a different point in the contract, as a condition for waiving his no-trade clause.
The meeting comes with the approval of the Marlins, in accordance with Major League Baseball's tampering rules; it could not happen otherwise.