If it's true that PSG will sign Messi, that means they have already stretched themselves to the max in terms of wages of players, only recently Neymar agreed a new deal, and of course he is on massive wages, they signed 3 experienced top players on free, so I suspect they offered them big wages, and let's not forget Mbappe will be offered a huge contract in case he wants to stay because Messi is joining.
This might mean they won't be able to sign Pogba on free next season, as he will command a huge salary and Raiola will demand a major agent fee, they still need to operate within FFP
This means only Manchester United can offer a good contract to Pogba, this now puts the club in a stronger position.
I might be wrong though but this what I think will happen if Messi signs for PSG