Article 1704 of the French Civil Code

If one of the co-permutants has already received the thing to be given to him in exchange, and he then proves that the other contracting party is not the owner of that thing, he cannot be compelled to deliver that which he has promised in counter-exchange, but only to return that which he has received.

Original in French

Article 1704

Si l’un des copermutants a déjà reçu la chose à lui donnée en échange, et qu’il prouve ensuite que l’autre contractant n’est pas propriétaire de cette chose, il ne peut pas être forcé à livrer celle qu’il a promise en contre-échange, mais seulement à rendre celle qu’il a reçue.