If Napoli wants to use Osimhen's transfer fee for Darwin Nunez, they should negotiate with Liverpool. They can also offer to pay in more than one installment. Liverpool might let the Uruguayan striker leave since he has underperformed in the EPL.
Napoli could make such a move if the deal with Galatasaray falls through but I don't think Liverpool are too interested in Osimhen because if they really wanted him, they would have negotiated a player exchange with Nunez and Napoli would also benefit by getting Nunez + additional money from Liverpool, or maybe Liverpool want to wait for Osimhen to become a free agent, I don't know, I think Osimhen situation is quite complicated because he is having difficulty leaving Napoli until the current summer transfer window.
I think Osimhen will remain in Istanbul because he likes to play there and Galatasaray wants him with them so much. The problem came with Napoli ambiguous attitudes. They don't want him in the squad and he will not play any game since he is out of Conte plans. Besides, Napoli should pay him a lot, like €24m next season while his release clause should be €75m. From what I understand, any offer above €51m is a win for Napoli! Meanwhile they refused an offer from Galatasaray of €60m so what's going on seriously with them?
Liverpool doesn't seem interested in Osimhen so far, otherwise they would make a move to grab him. The asked amount by Napoli is weird.