Him and Neymar together would be absolutely lethal. PSG can sell some of Krychowiak, Di Maria, Lucas and Matuidi to generate funds and comply with FFP. Hell, if they can get Mbappe, they could probably let Cavani go and that's £50m or so from Milan or another side.
Remember that transfer fees get ammortized over the length of initial contract. Neymar signed for 5 years so that comes down to £40m a year plus wages. Mbappe could sign for £120m for 5 years and that's £24m a season. Wages they could cover long term by selling a few players here and there too. A club like PSG would not make these signings if they weren't absolutely certain that no CL ban was imminent.
I still don't see this happening though. If PSG are getting another forward, it'll be Sanchez.