Or HaChaim on Genesis 25:31/אור החיים על בראשית כ״ה:ל״א

1 ויאמר יעקב מכרה כיום, Jacob said: "Sell as of this day, etc." The word "as of today," needs amplification. There are aspects to the birthright which do not become effective until after the death of the father, and as such they are legally considered as דבר שלא בא לעולם, something the eventual existence of which is still subject to doubt. The claim to such things cannot legally be established on an "if and when" basis. Jacob added the words "as of this day" in order to make the sale legal at this time. This is in accordance with Baba Metzia 16 and Choshen Mishpat 211,2. When someone who is poor and does not have what to eat goes hunting and offers the proceeds of his hunt for sale adding that such sale be effective "today," the sale is valid. The reason for this is that the poor man desperately needs the proceeds from such a sale. The Geonim write that the word "today" is not critical to the effectiveness of such a sale. Even if the sale was to be effective the day after it would still be valid. The decisive criterion is the critical need for the proceeds of the sale. The only reason the Talmud mentions the word "today" is to exclude dates such as "after thirty days or longer." As long as the time stipulated for the sale to take effect is within one day so that the seller can assure his survival during this space of time he has met the important criterion. When Jacob said "כיום," he did not mean "today" but "in accordance with the rule applying to sales that take effect within a day. He saw Esau was very tired and weak and providing this food for him at that time was a matter of survival. Had Esau not been in a temporarily desperate situation the sale would not have been legal.