Kaisipang Hudyo sa Mga Awit 128:3: Akeidat Yitzchak

(6) Esau's concept of the birthright and its values may not have differed much from the way many Jews perceive the "feast of Leviathan." Some are willing to sell their claim cheaply, whereas others treasure it and would spend their life savings to assure themselves of their share in it. Since Jacob realized that although he treasured the value of the birthright, Esau considered it of trifling importance, he felt morally justified in swapping it for a nominal amount. The apparent urgency to complete the transaction (sell it to me this day!) may have been prompted by the realization that the next time Esau would be exhausted he might well be at the point of death, and the opportunity for this transaction might have disappeared forever. Since Esau had made it clear that he had many close encounters with death (Genesis 25,32), Jacob made it plain that under such circumstances, when no documents were at hand to record transfer of ownership, the least he required was an oral declaration of a binding nature. The sale was to become effective "as of now!" The Torah gives evidence that Esau had an opportunity to reconsider this rash decision when he was fully restored to his senses, i.e. after having eaten. Therefore, we are told, "He ate, he drank, he got up and he went about his business; he despised his birthright."