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Osimhen's current market value is €120m, but I assume Napoli set a minimum transfer fees around €150m. It is surely a huge amount of money, only big clubs like Real Madrid or PSG that can pay the transfer fees. I also doubt that AC Milan or Inter Milan is interested to sign Osimhen, they may prefer to sign cheaper strikers. There are still many other strikers who have cheaper prices although the prices of most players are getting higher right now.
We don't know what the plan for Osimhen, but I think Napoli is better to sell him as soon as possible. I am not sure Napoli still can perform as well as the last season, Osimhen price will decrease if Napoli performance is decreasing. Unless getting a bigger money in the next transfer window, the top clubs may be not interested anymore with Osimhen. Napoli must let him leaving in the current transfer window if they want sell him at the best price.
I think you are right about the Osimhen situation, but Napoli seem more confident about keeping Osimhen than selling him. Osimhen can help Napoli to defend their title next season, but I doubt Napoli will repeat its success again without Osimhen.
In the end Napoli may need to accept another team's proposal for Osimhen if the player does not want to stay. I'm sure Osimhen won't be able to maintain the same form as last season, so Napoli should also consider selling him instead of having to apply for a longer contract extension. 120 million euros is a fantastic value, so Napoli has nothing to lose to release him to interested teams.