It's only the coach of a club that can understand the strength and the importance of the players in his team. The management does not knkw this. Flick knows how important Fermin is to the team which is why he didn't accept Barcelona to sell Fermin because he knows the problem that will pop up if he leaves the club. Flick has tried his best to build a strong team and Barcelona management wants to thwart his effort.
Flick's decision not to agree to the sale of Fermin was the right one, as Barcelona cannot afford to sell their current players carelessly if they want to strengthen their position next season. The team will undoubtedly feel the impact when a good player leaves, especially if Barcelona cannot then acquire another player of comparable quality to the one they sold. Therefore, not selling Fermin was a good move, and it's clear that Flick has seen the team's intricacies, which is why he immediately disagreed.