I am talking about bonuses, loyalty bonuses in particular.
It is a sum that used to be paid at the End of a player's contract, to encourage them to stay. It is often split over the length of the contract, if a player leaves, they get what they have accrued till then, but no more beyond that.
That is why players sometimes wait around on an agreed move, to collect thay year's bonus.
A legal precedent was actually set for this, when Neymar tried to sue Barcelona that they were due his loyalty bonus and the court ruled that they were not, as his contract was ended when he joined PSG.
Until someone can show me otherwise, that is all I know, and all I believe.