If Mbappe wants to gain personal benefits then a salary cut is something that is difficult for him to accept, but if he wants to achieve a much better career then taking the opportunity to take a pay cut is not a big problem. The Ballon d'Or and the Champions League trophy are the dreams of many players and Mbappe will get that if he plays with the right club and I'm not saying that only at Real Madrid because other players have gotten it before.
The opportunity for Mbappe to win an individual title with Real Madrid is greater than if he stayed with PSG, has large possibility for winning UEFA Champion League trophy with Real Madrid than PSG and he will get more support to be the best players when joining with Real Madrid. But Mbappe have ready with consequence about his payment salary, actually Madrid will reduce the payment salary received by Mbappe with PSG and he will get cut off few percent of his higher salary before.
Depend with Mbappe if want to win more trophies and get chance for winning individual achieving Madrid is the best club for him but he wants priority with higher payment salary he can extends contract with PSG for long term duration.