Even if they needed to raise the 167 million to pay for it, which they won't because of amortization and all the other accounting tricks, there would be no point selling Mbappe. In 5 years time he will still be worth 200 million, if he really kicks on it could be even more! Whereas its highly unlikely the same would be true of anyone else in their squad.
Plus I reckon if things got really really desperate the Qatari owners could find a million ways to get out of FFP. Chuck some money into a big chinese club or any club that doesn't have to worry about FFP really and sign those same PSG assets for massively inflated prices. Don't think that would leave UEFA a leg to stand on.