Article 220 of Civil Code

1 - A unilateral juridical act shall neither create an obligation nor modify or terminate an existing obligation, except in the special cases provided for by law. 2 - If the law provides for the creation, modification, or extinction of an obligation by virtue of a unilateral juridical act, the provisions of law generally applicable to contracts shall apply to that act, except those incompatible with its being based on a single will, particularly those concerning concurrence of the wills of the two parties to a contract. Public Promise of a Reward