Are there any UEFA/FIFA regulations regarding the payment of release clause in the January transfer window in European football? Are the clubs allowed to pay the release clause in the January transfer window?
As it's laconically stated in the FIFA Regulations on the Status and Transfer of Players - Article 16: Restriction on terminating a contract during the season:
A contract cannot be unilaterally terminated during the course of a season.
Which suggests that the sides (current club and the player) should bilaterally agree on the transfer during the course of the season (winter transfer window in most cases is in the middle of the season), as the activation of the release clause against the will of the current club is an unilateral termination.