I don't want to ruin your hopes, but you must be real in this case. You really expect Mbappe to be a free agent
I know is a very slim chance but again if the auction for his salary starts in the same year when Ronaldo leaves Juventus will have 80milion/year free on their cashier.
In a hypothetical world they can spend this budged all for Mbappè so hypothetically they can spend up to 60m salary to him and 20 for taxes.
Now, to equalize this offer other teams outside Italy need to spend 120m/year.
I know it will never happen but it is a dream.
Now, about the free agent.
If he doesn't want to renew his contract PSG can do nothing and they need to set him free also keep in mind when a player is at his last year he can sign for whoever he wants for free from 6 months minus 1 day at the expiration of his contract.
So he can sign for Real Madrid for example for free without any legal problem from 1st February 2020.
About taxes, I almost sure that premier is using the "old" Italian tax system too, it was the same in every major league.
If a players earn 8m/y you also need to pay 8ish m/y in taxes.