Any team would try to avoid spending more when they are in a financial crisis, what Barcelona did was for the good of the club and the player, if the player was willing to lower his salary, maybe this situation would not have happened. Gundogan is a great player, Barcelona really needs his energy to maintain balance in midfield, but maintaining the club's financial stability is much more important than retaining a player.
Selling players is not something that is considered bad, as long as both parties benefit, they will part on good terms. Barcelona are trying to get out of the problems they have faced for the past few years, but luck is still on their side as players from the La Masia academy are starting to develop rapidly. Clubs can save money on bringing in new players because they can be relied upon in the first-team squad.
The advantage for Flick was that Barcelona was already a strong enough team without additional transfers, Xavi tried to make it possible, and he wanted to strengthen the attack, he said this many times. To some extent, Flick did not need new transfers, but in the end, Barcelona let Felix go to free up money for such an opportunity.
And now they write that Gundogan is returning to City as a free agent, the contract is calculated until the summer of 2025, that is, for one season. This can be written down as a negative in Flick's work, he let such a player leave just like that, this is bad. But I probably understand why this happened, Gundogan wanted to work with Xavi, now that he is not in the team, he decided to return to City, I think he will do it with pleasure.