Not sure if contracts are (I'm assuming they are, or at least with the PFA in case of disputes?) but the deal sheet would be with the FA and the information would be on there.
What might happen is say if Chelsea sell Caicedo and one player to another team at the same time, and the Caicedo fee...
Surely transfer clauses work in a way they're designed to in a contract.
So it could be a % of the future transfer fee, it could be a % of profit, the % could change year on year as the player gets longer into their contract, the % could change depending on which club / league they've sold to...