Is it good business? Perhaps. You’re certainly right to note that it’s a hell of a profit on a 7m purchase in 2 years.
That does not mean it’s a good football decision.
40m is arguably a fair price for the player Guendouzi is right now, but selling promising young players before they mature is a classic ‘small club’ move.
Guendouzi is the kind of player we should be buying, not selling. The fact that we were ahead of the curve and got a great deal on him is precisely what means we should keep him, not sell him.
That 7m gives us the luxury of not caring about the player he is now, but the player he will be in 3-5 years.