Article 2243 of Mexico’s Federal Civil Code allows a party to promise to sign a contract in the future. Under Article 2246, the promise “debe constar por escrito” (must be in writing). It must also contain the essential elements of the final contract and be limited to a set time. If the promisor then refuses to sign, “en su rebeldÃa los firmará el juez” (the judge will sign in his place), under Article 2247.
A spoken promise with no date fails at least two of these conditions.