Napoli bought Osimkhen for the amount that amounted to just about 80 million, i.e. Formally, they did not earn anything on its sale. Plus, all these years, they paid him a large salary. The only thing I can’t understand is why Galatasarai made this transfer at all. Osimkhen would be a free agent in a year. Maybe they estimate his real value as 150 million and consider it a profitable transaction (expect to sell him in the future)? Otherwise, it is difficult to explain this record transfer for Turkish football.
If they didn't sign him and waited until he was free agent, Galatasaray wouldn't be the place he would go to probably.
Because in that situation there would be much bigger European teams going after him. Most likely from the place he wanted to be in too, the Premier League.
By the way Galatasaray can't sell him for even 80 million after a year. Because the Turkish league isn't prestigious. His value will drop quite much.
The Turkish Premier League with all its history and fanaticism is not the ideal place where one can sustain or improve the value of the players. Now, there is a huge difference between performing in one league and winning in the Champions League, so, by performing in just that league and not winning in the Champions League, Europe major markets will not be interested to shell out a massive 80 or 100 million on just one player. They will instead wait until the price goes down or they will wait until the player will get frustrated and request a move. It is safe to make a conclusion that this move is more risky than it should be.