What did Yaakov mean when he declared, "Sell me your birthright like the day"?
Rashi (citing Targum Onkelos): He meant that Esav should sell him the birthright clearly (with no strings attached), like the day is clear. 1
Ramban (interpretation of Targum Onkelos) #1: 'Sell me the birthright today, which you will inherit whenever 2 that might be.'
Ramban (interpretation of Targum Onkelos) #2: 'Sell me the birthright, whenever (on the day that) 3 you inherit it.'
Rashbam: 'Sell me today, your portion of the birthright of my father's property, for the money that I will give you now.'
Riva: In the future, when there is Avodah, one cannot sell it. Now that there is no Avodah, you can sell it. 4
Da'as Zekenim, Hadar Zekenim: Sell it to me for its value today, i.e. mere Tovas Hana'ah. Perhaps you will die before father, and it will be worth nothing to you!
Ohr ha'Chayim: Refer to 25:33:152:1.
Malbim: Like you despise it today, sell it to me. If one is about to throw his wallet to the sea, another may ask to buy it for a loaf and porridge. This is not guile; it is like saving from Hefker!
Ha'amek Davar: Sell it according to Yitzchak's status today. If his status will change, you cannot claim that the value of the Bechorah changed.
Gur Aryeh: Refer to 25:31:1.1:1.
Targum Yonasan seems to explain the Pasuk in similar fashion (See Peirush Na'ar Yonasan); see Targum Yonasan to 25:33 as well.
Rashi and the Ramban are arguing as to whether the word 'd'Lehen' (used by Onkelos) means 'clear' or 'whenever' (See Ramban, who elaborates).
As if the Torah had written (not "ka'Yom," but) 'ba'Yom.'
What did Yaakov pay for the birthright?
Rashbam, Malbim (to 25:33): Yaakov bought the birthright for money. Rashbam - he gave Esav the lentil stew as a sign that he had sold it - to clinch the deal, as it were. 1 Malbim - now that Yaakov was the Bechor, he was responsible for feeding people of the household.
Ramban: Yaakov actually purchased the birthright for the lentil-stew. 2
Why was Yaakov so concerned that Esav not retain the Bechorah?
Rashi: Because the Avodah (in the Beis-Ha'Mikdash) is through the Bechoros. Esav was not worthy of this.
Malbim (to 25:29): The Bechor was responsible for running the house to finance his parents. Yaakov was not concerned for inheriting a double portion. He wanted to inherit the Divine spiritual Berachos and Eretz Yisrael, since he saw that Esav is not worthy of this, and that Esav did not fulfill what is incumbent on the Bechor (refer to 25:29:1:2) and despised the Bechorah.
Maharal #1 (Netzach Yisrael Ch. 15, p. 89): Esav was physical, whereas the Bechorah is sanctity, disassociated from any physicality.
Maharal #2 (Gevuros Hashem Ch. 29, p. 113): Refer to 25:26:1.2:1, and the note there.
How can a Bechorah be sold? Halachah does not recognize the sale or acquisition of intangible items (Rambam, Hilchos Mechirah 22:13)!
Gur Aryeh: Esav sold himself to Yaakov regarding the birthright, along with all rights associated with it.
Also refer to 25:33:2:1, and the note there.
QUESTIONS ON RASHI
Rashi writes: "'Sell as this day' - As clear as day... a clear sale." Why is the word "day" added?
Maharal (Netzach Yisrael Ch. 15, p. 89): When Yaakov and Esav fought in the womb, they reached an agreement that Esav would receive this world, and Yaakov would receive the world-to-come. 1 The sanctity of the Bechorah relates to the next world; Yaakov was asking Esav to sell it to him, as they had agreed on 'that day,' i.e. in the womb.
Rashi writes: "'Your first-born [obligations] - The Avodah (sacrificial service) is performed by the firstborn; Yaakov said, 'This wicked one is unworthy to serve before HaSh-m!'" Is it possible for a Kohen to sell his status to someone else?
Mizrachi: Yaakov only wanted to prevent Esav from filling this role; not to become the Kohen himself. That is why he made Esav swear to ratify the sale. 1
Gur Aryeh: Today, a Kohen cannot sell his status, for the Torah has assigned it to him, and any other person is disqualified. 2 However, prior to Matan Torah, the Avodah belonged to the firstborn as an honor [as logic and propriety dictate that the eldest and most distinguished son should serve]. He was therefore able to defer that honor to someone else, just as a Torah scholar may defer the honor of the first Aliyah to someone else.