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Its part of standard football contracts Believe it or not I won’t loose any sleep
Here you go
An established professional who has, in the course of the season, appeared in fewer than ten per cent of the official matches in which his club has been involved may terminate his contract prematurely on the ground of sporting just cause. Due consideration shall be given to the player’s circumstances in the appraisal of such cases. The existence of a sporting just cause shall be established on a case- by-case basis. In such a case, sporting sanctions shall not be imposed, though compensation may be payable. A professional may only terminate his contract on this basis in the 15 days following the last official match of the season of the club with which he is registered.
Article 15:
https://www.fifa.com/mm/document/af...e_status_and_transfer_of_players_en_33410.pdf