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Napoli administration and boss are hard to deal with. I remember like 3 years ago, Victor Osimhen was at his top and worth more than €100m and Napoli didn't want to let him go under that amount. To be more precise they wanted higher than that, if I am not mistaken between €120m and €150m. Obviously no team is willing to spend that much so Osimhen remain in Napoli and we all know the rest.
Any ordinary team would have accepted to sell Osimhen even with €50m but not Napoli. They need a lot of pressure to let him go or wait to see him walk free after a wasted year in Napoli.
I think it's a very unpleasant situation for Osimhen. He and his agent probably don't want to negotiate with Napoli anymore after what he's been through in the last few seasons, so Napoli should be able to let him go without asking too much for his sale. But as you said, Napoli want more, they have the right, but of course it needs extra consideration until the transfer is realized.
Osimhen's current market value is €70m, which is still high, but several teams are interested in signing him. Liverpool and Al-Hilal are probably the most interested in him at the moment, but several other teams are also interested in getting his services. Napoli must act quickly, Osimhen only has one year left on his contract or Napoli will have to let him go on a free transfer.