Mendes does what is best for the client. He is his agent first and foremost. The Fati situation, as I understand it, was 50/50 at the time. He was the ultimate hope of Barca at that time. So the wage was in some sense justified by his previous performances and goals. The club, I guess, wanted to show support as well. But we know how it turned out.
What I am ultimately saying, is that Mendes is not only money centric. He could have sold Ronaldo to some clubs for more money. But ultimately helped him to get to United and to Real to leave a legacy. And hopefully he can communicate it to Yamal and his entourage.