I don't really know if that bonus that Mbappe would receive is really that big of a problem for Paris Saint Germain. In the end they are owned by a sheikh and everything gets paid by him anyway so that bonus payment should not be that big of a problem. I also don't think that Mbappe's main reason why he wants to stay another year at PSG is that bonus. As far as i know he is earning 72 Million € per season at PSG at the moment which makes him the best paid footballer in Europe by far. So he does not really need that additional money i would guess.
I think that Mbappe will stay another year because his primary target, Real Madrid can not buy him in this summer because they have already bought Bellingham and Real has pretty high debts already. So they need to wait until the end of the coming season to sign Mbappe.
The problem is not on PSG, of course they are able to pay all salaries + bonuses what I mean is Mbappe does not want to lose the bonus, he will get a bonus of around 90 million if he completes his contract while he did not want to activate the contract extension for the next 12 months (2025) this means he really wants to move to another club. If you are PSG owner what will you do? let him go for free or release it to another club at a high price. Don't forget this related to business, they can make decisions if there is no other choice.