It is actually depending on the board of Napoli itself. If they want to lower the transfer fees of Osimhen, it won't be so difficult to sell Osimhen. PSG agrees to sign Osimhen at €120 million, it is a huge money. If Napoli is serious to sell Osimhen, they must agree with €120 million. Unfortunately, it is rumored that Aurelio De Laurentiis wants Osimhen to stay in team, that's why he set huge transfer fees. As far as I know, he wants €150 million if PSG wants to get Osimhen.
If Napoli is serious to sign Lukaku, they won't pay for a very expensive price. Chelsea only sets a minimum price at €29m - €30m. I think Napoli has the money, I don't know why they must sell Osimhen to sign Lukaku. If they want to sell the current players, they can sell other players. There are Giacomo Raspadori and Giovanni Simeone who can be the options to put on the transfer list. If Napoli succeed to sell these players, it is enough to pay the minimum price of Lukaku.
I doubt if Aurelio De Laurentiis doesn't want Osimhen to leave Napoli. The club doesn't want to sell Khvicha Kvaratskhelia and they made it clear that they wouldn't entertain any bid from any club. But they have made it clear that Osimhen is for sale. The club's target is to raise money from his transfer that will enable them to fund their transfer plans. They already know that the Nigerian striker is no longer comfortable paying in Napoli and wants to leave at every good opportunity.
Napoli doesn't want it reduce the asking price for Osimhen but they want Chelsea to sell Lukaku at a discounted price. Lukaku cannot replace Osimhen because both of them are not in the same class. Lukaku has passed the prime of his football career while Victor is in top form and still can improve more if he plays under the right coach. Napoli will still need a younger striker that will serve as a backup to the Belgian international. Osimhen will be glad to leave Napoli because he has had a sweet and bitter experience.
lukaku will never be able to replace Osimhen for the simple fact that to score all those goals you simply have to do them and lukaku has been a static player for too long and is not capable of scoring all those goals now if Conte likes them they will surely take him but they won't take him I see myself capable of such an adventure