PSG are in an awkward situation actually. They want to keep Mbappe with them or simply let him go with an awesome reward rather than seeing him walk for free next season.
It is known that PSG is pushing for an extension of contract with Mbappe, but it is clear that he will not do that since he counts on joining Real Madrid. Basically, PSG will be forced to sell him once they find a good substitute.
Mbappe has stated that if he is not interesting to join in another club this season but it's not for upcoming season. Even though if PSG will be forced to sell him but without personal agreement and it will be useless.
It's caused by there must be a personal agreement before the transfer will happen. It has become one of main requirement to make the transfer happen.
The club can't sell him if mbappe is not agree on it. PSG has two choices, keep him or terminate his contract. It's a way for mbappe to force PSG to terminate his contract.
I think that this can be considered as a pressure from mbappe to the PSG. Mbappe knew how the transfer process is working and that's why he was putting PSG in a hardest situation.