I think they were certain Donnarumma would leave the club at the end of his contract and wanted to secure his transfer fee. They experienced this problem with Mbappé. They let go of a player who could have earned a very high transfer fee. Kylian Mbappé was transferred to Real Madrid on a free transfer after his contract expired. They don't want to experience the same thing with the Italian goalkeeper
The thing with Mbappe back then was that Mbappe doesnt want any other club except Real Madrid. PSG doesnt want to let him go not because they dont want him to go for free but because they want him to stay at PSG however Donnarumma has different case compared with Mbappe. PSG doesnt want Donnarumma anymore so he is free to either sign with another club or stay for another year then leave for free
Either way, pretty sure that he will be signing with a club before the transfer window is closed. Its just weird if no other clubs want a top keeper like him on their team