But it is only a theoretical offer - that PSG seem happy to accept is largely irrelevant, I mean why wouldn't they. Whether or not it turns from something purely hypothetical onto something real is entirely dependent on whether Mbappe is interested in stepping aside from serious football for a year at the height of his career.
PSG would be foolish not to accept such an offer unless they can extend his contract. If they don't accept such an offer and they don't extend his contract, he can walk away next year for nothing. They paid €180 million (rumoured) for him. €300 million would be a good return and could finance some really good players.
While his agent may be rather keen on the deal as they will also make a fortune, there are likely to be wiser heads cautioning that clubs will note that this move would make it clear that he's more interested in money than actually playing for a club and winning trophies.
It's just business. Players are often putting money ahead of club. Don't forget top tier players are accustomed to lavish lifestyles and will be retiring in their mid thirties. They need to accumulate a lot of wealth during their careers in order to continue to live in the style to which they are accustomed. Players often make decisions that are bizarre from a footballing perspective but make financial sense. For example, anybody moving to Real Madrid (or previously Barcelona) who isn't absolutely exceptional risks never getting a game. They do it anyway for the pay cheque.
Those wise heads will also caution that a year playing non competitive football may render him off the pace and not properly match fit for the Euros next summer. So by summer 2024 we could have Mbappe, whose performances have simply vanished below the radar for a year, who disappoints at the Euros and is clearly known (by his actions) not really to be interested in playing for a big club rather than for the money.
It might temporarily take the edge off his ability but it wouldn't be as bad as a player coming back from a long term injury and that happens routinely. If after a year he comes back with a price tag of say €150 million, there will be plenty of clubs lining up to sign him. He'll still only be 25 at that time.
And let's not forget that Mbappe turned down the chance to join Real Madrid last year - presumably because his head was turned by the money. So it is entirely possible that Real's interest will go cold for a player who has already turned them down and shown that he isn't interested in maximising the chances or winning big club trophies at big clubs at the peak of his career.
PSG is, or at least has been as much a vehicle for winning trophies as Real Madrid. They haven't won the ECL yet, but you wouldn't bet against them.
Of course, there's another possible motive for staying at PSG and that is that it is a French club and Mbappé is French. Footballers have made decisions based on geography in the past.
Err Erling Haaland just to name one.
Haaland is not in Mbappé's class. He's a top striker but he needs a good team behind him to provide service (Norway - Scotland shows us what happens when he doesn't get it). Mbappé is a much more complete player.
Side note: Haaland is qualified to play cricket for Yorkshire.