FIFA don’t decide, they issue and process the paperwork. I suspect Messi is just going to sign for City and Barca will take him to court. The court will rule whether he was within his rights to leave for free due to Covid extending the season. If he wasn’t they will look to see if the £700m release clause expired in the 4th year of his contract (this seems cut and dry as the exact working in the contract has leaked). If there’s no clause but he shouldn’t have left for free, the court will decide an appropriate fee taking in to account his age, wage, profile and probably Ronaldo’s fee as he also moved at 33 and matches Messi’s profile.