Basically Galatasaray would convince Liverpool instead of Napoli 
Like not even pay Napoli, just pay Liverpool, and get Nunez to Napoli and Osimhen to themselves that way lol. Because we all know, whatever Galatasaray pays, will go to Nunez probably, maybe a bit more money could be left.
Last I saw from twitter, it's 5 year deal for 75 million, which would be 15 million a year, not a lot of money at all, definitely could be paid by Galatasaray considering it is not 75 million suddenly, just 15 upfront, and next 15 is next year, so it's fine. With that kind of payment deal, they can get him. Also, Liverpool could end up doing this fine as well, doesn't feel like it would be that bad, both of the situation a lot better for all three teams.
Your scenario only happen if Liverpool keen in accepting installments, but they also wanted direct cash like Napoli. So that scenario most unlikely to happen. Five year installments is very long in football. I don't think Liverpool is keen in accepting that long term agreement. Like not even pay Napoli, just pay Liverpool, and get Nunez to Napoli and Osimhen to themselves that way lol. Because we all know, whatever Galatasaray pays, will go to Nunez probably, maybe a bit more money could be left.Last I saw from twitter, it's 5 year deal for 75 million, which would be 15 million a year, not a lot of money at all, definitely could be paid by Galatasaray considering it is not 75 million suddenly, just 15 upfront, and next 15 is next year, so it's fine. With that kind of payment deal, they can get him. Also, Liverpool could end up doing this fine as well, doesn't feel like it would be that bad, both of the situation a lot better for all three teams.
Installment payments for player purchases are common, but a five-year term might be too long. If Napoli is interested in Nunez, I think Liverpool will reach an agreement directly with them.
They are savvy businessmen, and they know which offers will be profitable and which will also be detrimental. I won't say this scenario is impossible, but it's difficult to achieve.