Genèse 25:31

And Jakob said, Sell today, as (on this very) day, what thou wouldst hereafter appropriate, thy birthright, unto me.

SELL ME THIS DAY (‘KAYOM’) THY BIRTHRIGHT. “I.e., as this day. (Explain it as the Targum rendered it), ‘as this day.’” Rashi now proceeds to interpret the Targum to mean, “just as this day is certain, etc.” Just as this day is certain, so make me a binding sale.” This is Rashi’s language.
The literal meaning of the word kayom is “at this time,” just as: But stand thou still at this time (‘kayom’), that I may cause thee to hear the word of G-d; At this time (‘kayom’) you shall find him;, Verse 13. Let the fat be made to smoke at this time (‘kayom’);, 2:16. But unto us belongeth confusion of face, as at this day (‘kayom’).
It would appear from the opinion of Onkelos, it would appear from this, etc. that because the sale of the birthright was to take effect after the death of his father Isaac, he [Jacob] said, “Sell me the birthright, with the sale to take effect on whatever day [our father’s death] may occur.” This is a typical usage of lahein in the Aramaic language: “Wherever (lahein) are you going?”. This means, “To what place are you going?” This is derived from the expression, “Whatever (hein) you let me know.” Similarly in Bereshith Rabbah, Parshath Vayishlach, “Wherever (velahein) are they going?”. See, however, Theodore’s edition of this Midrash, p. 906, where he quotes from manuscript, velahein, as Ramban has it. is derived from the expression, “Whatever (hein) is broken.” This is their customary usage of language in many places. And in the book of Daniel this form appears with a patach. under the letter lamed, similar in meaning to the word ilahin (which): Which ‘lohin’ the angels whose dwelling is not with flesh; whatever (‘lohin’), O king, let my counsel be acceptable unto thee., 4:24. Now Onkelos translated the Hebrew word zulathi as ilahin (excepting Caleb) is translated by Onkelos as elahin Caleb. To the Aramaic root lahin which appears in the book of Daniel — (see my Hebrew text, p. 145 line 2 from bottom, covering Notes 53 and 55 here) — Onkelos added the letters aleph and yod, thus making it elahin. The intent of Ramban is to indicate that it should not surprise us that Onkelos added a dalet in the text before us, thus making it dilhein, for just as the original word hen was augmented to become lahein …

מכרה כיום, meaning: “immediately!” Sell me your share of the birthright which you are entitled to from our father’s estate which I will give you. After that I will give you to eat as proof of the sale and as confirmation. We find a similar transaction in Genesis 31,46 where Yaakov and Lavan ate in order to confirm their non-aggression pact.

מכרה כיום, seeing that as of now all your interests are focused on your work so that you are so tired that you cannot even recognise a dish of lentils for what it is, there is no question that you would not have the time or energy to perform the duties involved in the obligations associated with being a firstborn

ויאמר יעקב מכרה כיום, the letter כ in the word כיום is known as the כ האמתי, a formulation we also encounter in Samuel I 9,13 כי אותו כהיום תמצאון אותו, “for truly today you will find him.” Other examples of this prefixכ being used to emphasise the truth of something are found in Numbers 11,1 and Proverbs 10,20, as well as in Hoseah 5,10. In our verse the letter כ means that Yaakov did not want Esau to sell him the birthright symbolically, but that he meant for a fully fledged legal transaction to be completed, his making a proper kinyan acquisition. [I believe this supports the view of our sages that what was at stake was not Esau’s share as a firstborn in an extra third of his father’s estate, something that at this point was only a potential claim as opposed to an actual claim, and therefore could not have been transferred legally in a binding manner. What was at stake were obligations a firstborn is duty bound to fulfill from the time he is physically and mentally capable of doing so. Yaakov felt that Esau was spiritually not equipped to perform such duties, hence he wanted to relieve him of them. Yaakov could not acquire the spiritual aspects of the birthright merely by pointing out to Esau that it was a burden for him anyway, as the position of the firstborn was something firmly entrenched in the culture of his time. Seeing that the world at that time was concerned with material wealth primarily, Yaakov had to compensate Esau according to the terms other people would understand as being appropriate. Ed.] In spite of Yaakov compensating his brother Esau in “real” terms for something to which his claim was thus far only potential, he was punished for initiating such a sale because he had upset the norms of civilised society by doing so. The fact that Esau rose to great prominence and distinction and hardly seems to have suffered any disadvantage for the deal Yaakov offered him, did not exonerate Yaakov from trying to upset the norms of society. [this view, i.e. that Yaakov paid Esau with money, not with a dish of lentils, is supported at least in part by Rashbam. Ed.]

מכרה כיום, “sell to me as of today!” Yaakov refers to the value of Esau’s birthright as of that day, and offers to pay that price. Seeing that the value is completely potential, Esau not knowing if Yaakov will outlive him or will outlive his father, the price must surely be minimal. In either of those two events the birthright would be completely without any value.