This article examines the legal nature, triggering events, and calculation basis of sell-on clauses in professional football player transfer agreements through the jurisprudence of the Court of Arbitration for Sport (CAS). It focuses on a central interpretative challenge in contemporary transfer practice: how sell-on clauses should operate when subsequent transactions depart from the traditional fixed-fee permanent transfer model and instead involve deferred, conditional, or downstream economic elements. The study adopts a doctrinal legal methodology and qualitatively analyses 23 CAS awards rendered between 2006 and 2025 within the framework of Swiss contract law. The analysis is structured around three interrelated dimensions: (i) the legal nature of sell-on clauses, (ii) the identification of the triggering event, and (iii) the determination of the calculation basis. The article demonstrates that sell-on clauses function as economically integrated components of the transfer consideration rather than as ancillary contractual mechanisms. CAS jurisprudence places significant weight on the economic substance of subsequent transactions while treating contractual wording as the principal interpretative constraint. The article concludes that the effectiveness of sell-on clauses depends less on abstract doctrinal classification than on precise drafting capable of capturing the economic structure of modern football transfers.

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sell-on clauses,football player transfers,contractual interpretation,triggering events,calculation basis,Court of Arbitration for Sport (CAS)