Osimhen is drawing the attention of other teams by a consistent performance this season for sure. He scored 9 goals in 11 games he played in the league already. As far as I know he doesn't have any release clause now which is good for Napoli. Because if he had that the amount would have been quite low compared to his value right now most probably. It would have been similar to Dortmund's losing Haaland to Manchester City for only 60 million euros.
Napoli would like to extend with him to fend off the interest in him. They might like to add a release clause worth no less than 100 million euros as well. But I have big doubts about Osimhen to accept this. He might be willing to sign for a bigger team in the future.
I don't think, Napoli manager will release Osimhen now, because they have seen what happened to Dortmund and Liverpool that sold their potential strikers with a huge amount of money that is causing the team to experience poor performance in this season. Osimhen has showcased some skills that is making other teams to use any amount of money to sign him into their team, because he is a young talented striker that has more strength to play very well. I think, Manchester united will like to have striker like Osimhen to replace cristiano Ronaldo so that their performance will remain stable after the FIFA world cup.
It is not up to the manager, it is based on the offer they get. If they get an offer like lets say 80 million or something, they would not be able to reject it, imagine him getting injured and a big one as well later in the season, that is not money they can gamble with. The whole teams salary is less than that, they would be losing a chance to make so much money and they would take that deal. It's up to the management to decide to sell him or not.
In this case, it's worth selling him and losing the title than keeping him and get the title with a possibility of him injuring himself and they end up losing the title anyway. That risk doesn't worth the possible reward.
I think that the Napoli coach should understand that if they let Osimhen leave Napoli right now, they are very likely to regret that very soon. He is sure to become a better player by moving to other elite clubs. And by that time his transfer market value will have increased as well.
The most critical thing is if they do let him depart right now they will probably lose track of winning the league title which they are trying to do right now.
Rght now I think they are on the right track to winning the Italian League title.
Probably one move from them is going to be selling Osimhen and getting Cristiano Ronaldo. But I do not think that is going to be an optimal move for them.