Personally, I hate when the selling club requests a Percentage from a next sell as probably I will sell players for big money and on a long run it will probably be better to pay a bit more when buying the player instead of giving 20% or more from a future transfer. The problem is that these clauses are ticked as non-removable at a lot of times when I was trying to buy young, promising players. In the previous versions of Football Manager, I would need to wait another six months until I could get rid of the clause
But pretty early in FM19 I found a hackz that allows to remove such a clause pretty fast. So now when I get such an additional clause, I just follow these steps:
Have fun xoxo