The great thing about the equity idea is that it ties him to the club and/or CFG long term.
Beckham left MLS in 2012, but he's been an ambassador and spokesman for MLS for the past 8 years because it's growth was good for him.
Someone compared Messi to Michael Jordan and the Bulls, and I think that's a good comparison, Messi is going to be making people money for decades after he's retired. If you can tie City to that not just for 3 seasons as a player, but for a further 5 or 10 as some sort of City ambassador or even just as a very powerful ally, that'll be hugely profitable.