(1) A unilateral promise to sell or purchase shall become effective as a sale from the time when the other party declares his intention to complete the sale.
(2) Where no period is fixed for the declaration of intention mentioned in the preceding paragraph, the person who has made a unilateral promise may give a peremptory notice to the other party to give a definite answer within a reasonably fixed period as to whether he will or will not complete the sale.
(3) If the other party fails to give an answer within that period, the unilateral promise shall become void.