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.
Napoli earns nothing from Osimhen sale, but the truth is that they get his service for several years + scudetto in exchange. They also booked their profit from Osimhen's future sales. In conclusion, they still in the profit.
As for Galatasaray, it's easy to understand what's their motive to agreed on such weird deal to get Osimhen. He scored 26 from 30 games, which was awesome. So no doubt they have strong will to make their scoring machine as permanent transfer.
Although the risk is very big for them consider he may be underperform anytime, they're ready for that.