Come on.
This is on the club, not Messi. They're the ones that handed out absurd contracts to players who wouldn't even get half of that anywhere else in the world.
He agreed a 50% pay cut. Why should he play for free when you've got other guys there in the dressing room that won't budge?
He definitely shouldn't play for free especially when he's been about the only good thing about that club for a few years, but if it was important enough to put him in floods of tears, he could have negotiated one year at whatever rate gets them lined up with limitations with an agreement to review next year.