What's the last good player Napoli let go to a league rival?
Napoli is a well run club that cares very much about the books and accounting. Napoli fans hate De Laurentiis for the same reasons our fans hate Gerry, they think their presidents don't want to spend money. That's why Conte started complaining in August that Napoli didn't get the signings they needed and threatened to quit (like Maldini).
Napoli did not want to make those moves until they sold Osimhen first, because they desperately needed those funds to balance out the incoming transfers.
After Conte's fiasco, Napoli's management bent the knee to Conte and bought the players they needed in hope that Osimhen will eventually leave in the following weeks. No club was willing to pay the asking fee nor the wages Osimhen wanted, and hence both Napoli and Osimhen were in emergency crisis mode on 1st September because the transfer window shut and neither of them got their move. Osimhen was going to be permabenched and Napoli are heavily in the red (they still are, look at their financials). They managed to loan him to Galatasary as long as they paid his wages (10m) in full but the problem is going to return next year.
After what they went through, they will surely sell him to whoever as long as it benefits their balance sheet, and it will.
Besides, they sold Higuain to Juventus (but I think there was a clause) after he scored the most goals ever for a striker in Serie A under Sarri.
