It's more a matter of principle. Paris Saint-Germain does not need money as much and is not in such a hopeless situation regarding financial fair play as Barcelona, which urgently tried to find buyers for some of its players, so PSG can afford to do such symbolic things, such as, for example, we are not against selling a player, but only on their own terms. However, I am not at all sure that Real Madrid will agree to the proposed conditions, especially since the president of Real Madrid, Florentino Perez, expressed his dissatisfaction with Paris Saint-Germain's rude response to the transfer offer. Real Madrid seems to be ready to increase its offer, but I don't think that it is by the required amount.
I do not think that PSG will be selling Mbappe, I believe that they are going to let him go because there is no money in the world that worths Mbappe and it is not about selling him, it is about basically keeping him and showing that they care about him so much that they even declined 160 million offer. You think that does not have any impact on Mbappes decision to stay or not? Guys were offered 160 million and they said no, which means that Mbappe knows that PSG wants him this badly, and that will play into the fact of how much he will want when contract is offered again.
I believe that given enough money PSG could still hold him, offer him twice what Real is offering and he will stay, who would say no to 2x amount of money? And give him like 5+ years worth of contract, give him like 50 million a year for 7 years, and I do not see any case that he is not staying, that is A LOT of money.