If Real Madrid only offered a salary of 21 million pounds it would be a very far salary reduction. because Mbappe's current salary at PSG is 72 million euros. so it would be a very far nominal salary reduction.
Of course Real Madrid cannot pay Mbappe what he earns in PSG, they do not overpay players the way the French champions do, PSG do not have the kind of structure that Real Madrid has, they overpay in salaries, wages and bonuses and that is not how a well run club should operate. If Mbappe wants to play for Real Madrid, which is arguably the biggest club in the world, he knows within himself that he must take a paycut.