I am not sure that the transfer of Mbappe will happen in the next transfer window, because Paris Saint-Germain, unlike for example Barcelona, has no problems with either finances or financial fair play and therefore can afford to play a risky game and not sell Mbappe now, hoping to convince the player to stay in the team and extend the contract later.
The point isn't about the money or financial situation of both clubs, but Mbappe seems no longer interested to stay in PSG. If PSG cannot convince him to sign his contract extension, then probably PSG will get nothing if Mbappe decides to leave in the next transfer window. In this situation, I assume PSG is better to sell Mbappe in this transfer window to have some transfer fees. PSG also can start to plan for a replacement for Mbappe as soon as possible. This will be effective than spending time to force Mbappe to stay.