These extra amounts certainly add up. Take Manchester United as an example. In their third-quarter financial results for 2024-25, announced 6 June, United confirmed that "under the terms of certain contracts with other football clubs and agents in respect of player transfers, additional amounts, in excess of the amounts included in the cost of registrations, would be payable if certain substantive performance conditions are met". As of 31 March 2025, the potential liability was £138.82m. Of that, £112.25m related to player appearances, team success or new contracts, £3.19m to international appearances, and £22.03m to 'awards'.

When Anthony Martial signed for United, one add-on was connected to the France striker winning the Ballon d'Or – never realised. United also rejected speculation Mason Mount had triggered an add-on from his Chelsea transfer by playing in a European final. The Martial clause was far more difficult, though United argued they would happily pay if the forward reached that level, given the on-pitch benefits. This article is from BBC Sport's Ask Me Anything team.