In sports agent contracts (exclusive common interest mandates), if the intermediary’s remuneration clause is not precisely drafted, it must be declared null and void. As soon as a second agent contract is concluded between the parties, this deficiency is resolved; the extension amendment to the first agreement, declared null and void, which is not a mere accessory to it, remains fully applicable. In such circumstances, a footballer who extends his employment contract with the assistance of a new intermediary before the end of the contract binding him to his first agent stipulated in the amendment, without justifying a legitimate reason for early termination, is required to comply with the penalty clause contained therein.