Donnarumma reportedly demanded a pay rise, but PSG management was reluctant to agree. Other reasons, Luis Enrique is reportedly dissatisfied with Donnarumma playing style, although he admitted that letting him go was a difficult decision. There may be a lot going on that we don't know, but it's clear that several clubs are very interested in signing Donnarumma. We'll have to see where Donnarumma goes, and it must be admitted that the way PSG treats him is disappointing.
If he demands a pay rise from the team, it certainly won't be immediately agreed to, as PSG also has significant salary obligations for other players. Furthermore, PSG's players have improved significantly after winning the UCL last season, so PSG must find a suitable replacement if Donnarumma is their final option. But why do you say letting Donnarumma go is a difficult decision for PSG, given that he is being targeted by other teams?
There will always be pros and cons regarding player transfers, especially for players who have just helped their club win a trophy. Many may not know the background that made Donnarumma leave PSG.
Regardless of whether the coach is satisfied with his performance or not, I believe Donnarumma has given his best. If PSG treated him poorly, I think there are other issues that are definitely not disclosed to the public. What circulates might just be speculation from the media.
If Donnarumma has a bad personal relationship with the manager or management, then he may decide to leave the club, but he may leave PSG and go to another club or stay at PSG, but we have to accept that this goalkeeper has performed well for PSG. If Donnarumma wants to leave the club, then PSG may want to sell him to another club before his contract with PSG expires. According to the rules of the transfer market, if a player goes to another club as a free agent, then the current club of that player will not get any money for that player's move to another club. In that case, if Donnarumma leaves the club as a free agent, it will be a big financial loss for PSG, so naturally PSG will not accept this huge financial loss.