Basically, Napoli wants to cash in on Osimhen to get a big profit, as they did previously with kvaratskhelia. However, Napoli maintained their principle of not releasing Osimhen to domestic rivals, fearing that he would have a significant impact, making it difficult for Napoli to retain the Scudetto. At first glance, the anti-Italy clause might seem harsh, but Galatasaray had no objections to the agreement. Napoli also included another clause, they will receive around 10% profit in the future should Galatasaray sell Osimhen to another club.
https://onefootball.com/en/news/osimhen-to-galatasaray-deal-done-eye-watering-fee-and-anti-italy-clause-41402603Kvaratskhelia didn't spend much time in the market as Osimhen, it's difficult to get a clinical all round striker like Osimhen which is an advantage to any team that owns him. If Napoli forgets, Galatasaray are playing in the Champions League as well, peradventure if the league draws a fixture against both sides, Galatasaray can't play against Napoli without the striker. All of these are happening because of the contract, i hope his agent learns how to communicate before agreement. The sell clause part was unnecessary after everything that happened before the agreement.