They wouldnt let him leave for free most likely and while not needing the cash they will want the sales towards FFP and allow them more freedom.
Worst case scenario for PSG is both Neymar and Mbappe leave around the same time or a year apart for little money.
Maybe they gamble on going all in on CL and accept the FFP hit but cant see it.
It would be totally ridiculous and irresponsible from PSG ot to recoup some money invested to buy Neymar and Mbapp?. However, even if they let them leave for free, they have no problem with FFP. Them leaving means we don't have to pay their big salaries and the remaining part of their transfer fees. Neymar cost PSG 80 M a year without his primes. Enough to invest and not be in trouble with FFP.
Selling means we will have even more room, not selling means we got no cash but usually it is not a problem with Qatar.