In the summer of 2023, right after his best ever season and Napoli winning the title he was offered insane money from Saudi and there were offers from various clubs in Europe. Napoli didn't want to sell him and he didn't want to leave yet. The contract was due to expire in June 2024 (salary at 6M net, their salary cap back then), he got a big increase which kept him at the club (valid through 2025). It was reported to be 10 M net guaranteed (and 2M more with bonuses). Over the 23/24 season he didn't get the bonuses, as they finished 10th and he was looking at a season with Napoli not in Europe, that's why he asked to leave.
During the summer of 2024 Napoli couldn't find an agreement with the Saudi club that was after him (Or Napoli tried to rinse them...) and Osimhen couldn't get an agreement with Chelsea (Osimhen's favorite buyer at the time). After the deadline, Osimhen decided to play for Galatasaray who would pay him 12M, Napoli agreed to loan him there and offered him a new contract so he would come back after the loan and still be an asset for them. He signed that contract, worth 12M net apparently (actually Napoli has an option to extend to 2027 but the salary would go up to 15M net // Which is also the offer put forward by Galatasaray now). In exchange for the last extention, Napoli gave him a release clause of 75M in the new contract, making it easier for clubs to buy him. I don't know if Galatasaray can afford the 75M and if they are willing to pay it for a 3-year contract, because that seems to be part of it. He doesn't want to commit to a very long deal...