There's also a clause preventing Osimhen from being sold to Italian clubs for two years. Osimhen won't be able to play against Napoli. Of course, there's no such clause. The transfer wasn't fully finalized until the signings were made, but it was breaking news. It was a major payment for Galatasaray.
It's understandable that Napoli don't want to send Osimhen to their rivals. I think Juventus wanted him but Napoli wanted 85 million Euro. For now, at least they will be comfortable for 2 years. Galatasaray didn't buy him just to win the local league title. They need to advance in the Champions League. But it's very difficult considering last season. Last season they had Osimhen but they played very badly in European games and lost even against very weak opponents. They need to change that this season, otherwise all this investment will be for nothing. Not much reward for the champion of the local league. Champions League matches should be the main target. Even winning single match brings a serious income.