55Furthermore, as regards the link between the action for a declaration of nullity and the recovery of sums paid but not due, it suffices to note, as the Advocate General pointed out in point 80 of his Opinion, that, if there had not been a contractual relationship freely assumed between the parties, the obligation would not have been performed and there would be no right to restitution. That causal link between the right to restitution and the contractual relationship is sufficient to bring the action for restitution within the scope of matters relating to a contract.
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