It is common practice these days for transfer fees to have performance-based add-ons. The theory is the selling club gains more if the player goes on to be extremely successful, while the buying club is to some extent protected financially if the transfer doesn't work out. In some cases, the total value of add-ons is significant - Real Madrid could end up paying an extra £30m to Borussia Dortmund for Jude Bellingham depending on how successful he is over the course of his career in the Spanish capital, for example.