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.