Messi will pay for his own wages.
The club already spoke to commercial partners to see how much extra they'd pay with Messi at the club in 2016, and there'd likely be renegotiations with a lot of them, exactly as Juventus renegotiated their deals with Adidas and Jeep when Ronaldo joined.
Then there's the next shirt sponsor. Shopping about the chance to have your brand on Messi's chest every week, and have him in you advertising is basically writing a blank cheque, IMO we'd be looking at rates near United's £75m, Real's €80m a year.
The other players aren't stupid. They will see these deals being done within months of Messi arriving and they'll know that's why he'll take home £80m a year all in. They will see the 10s of thousands of Messi shirts on kids backs all over the world.
That's before they even step on the training pitch with him, and realise how much better he is than them. Some of the greatest players of the last 20 years - Aguero, Iniesta, Xavi, Henry, Ronaldinho - they all say he's on a different level to every footballer they've ever seen or played with before. They know why he gets paid the amount he does.
Barca's wage problems came with Neymar. He had to get paid because of his commercial pull and how much they competed for his signature, but then Suarez wanted to be near him, then Griezmann, then they killed their wage structure.