Sell on clause is simply the percentage you include into a players contract which gives the parent contract the right to some amount of the transfer fee of the new club decides to sell in future, like if it is included into osimhen contract and Galatasaray gets to sell him off later they will give some percentage which was agreed earlier to Napoli. For example Manchester United sold off Carracas to Benfica and included a 20% sell on clause and now that Benfica is sold him off to Real Madrid for €50m Manchester United will be pocketing €10M. Same thing they did for the Elanga dela deal and also got 5.5 million from the deal too. But in this Napoli deal it is certainly not a good option.

My reason is sell on clause or any other clause is usually included in a contract of a player if the parent club is actually wanting the player to get more minutes outside and also sees much potential in the player and also of the transfer fee is actually small. It is acceptable by the buying club because it comes with lower transfer fee but in this case Galatasaray are actually paying almost the release clause of Osimhen which is huge so it is not a good deal because it technically means they don’t still fully own the full right of the player.