Apologies if this should be in the Finances section rather than Transfers, but it could be in both.
When a player is sold, FIFA regulations state that 5% of the transfer fee should be withheld and distributed proportionally to clubs who have trained the player between the ages of 12 and 23.
Although solidarity payments are in the game, they are not accurate at all and I can't even make sense of what rule it is that is trying to be replicated.
If you are aware of this and just having problems replicating it in the game then thanks very much and good luck.
If you are either not aware of this, or you thought it was replicated accurately, then would you mind letting me know and I will provide you with some more accurate info including relevant saves.