Napoli knows that if they fail to sell Victor Osimhen this season, he will leave as a free agent when his contract ends. Since Napoli doesn't want to accept Galatasaray's bid so that Victor Osimhen will move to Galatasaray this season, I agree with you that Napoli wants to destroy Victor Osimhen's career by making Victor Osimhen's transfer deal difficult for Galatasaray so that Victor Osimhen will remain at Napoli till next season without a guarantee of a first-team match.
Napoli can't ruin Osimhen's career, he's only 26 and even if this season is uncertain and no one can buy him, he'll spend another year on loan and Napoli won't get anything because in a year he'll be a free agent and can choose the team he wants. So now it's in Napoli's interest to complete this transfer to Galatasaray and earn 75m. I think they're just trying to get the best deal and get the money as fast as possible and Galatasaray isn't the richest club and can't afford it.