Bereshit 25:29,30 record that Yaakov was boiling some type of soup and Esav asked to have some since he was weak. This would appear to be a reasonable request from one brother to another, but Yaakov gave the surprising answer that he wanted Esav to sell him the birthright for the soup, 25:31. Esav agreed to this deal since he stated that he was on the verge of dying, and if he was dead, then there was no point to having the birthright, 25:32. This agreement was not enough for Yaakov. He made Esav swear to ensure that Esav would not back out of the deal, and when Esav agreed to swear, then Yaakov gave Esav the soup, 25:33,34. The episode ends with a note by the Torah, that Esav despised the birthright, 25:34. Why did Yaakov not freely give the soup to Esav? What did he buy when he bought the birthright? What is the significance of this transaction? Was this just another example of their fights (see comments by David Hoffmann)?
One approach (see Samet, 2004, pp. 111-113) is that the birthright signified who was to receive the blessings from Yitzhak. This approach explains why Yaakov wanted the birthright since he wanted to receive the blessings and also the significance of this episode, that Yaakov was truly the brother who should have received the blessings of Yitzhak. With this idea, Yaakov thought that he had to get Esav to sell the birthright even if it meant taking advantage of Esav.
This approach is difficult. Why would Esav despise the birthright if it implied which brother was to be blessed? Was he really so reckless? Even if Esav was impulsive, it seems that both of the participants to the deal did not believe that their transaction related to the blessings. When Yitzhak attempted to bless Esav, Yaakov never attempted to stake his “rightful” claim that he had bought the birthright and instead he pretended to be Esav. Also, after Yitzhak was tricked, Esav stated that Yaakov had deceived him twice, once by the birthright and once by the blessings, 27:36. This duality implies that Esav viewed the birthright and the blessings as two separate issues.
A second approach is that the sale of the birthright relates to the financial question who would inherit the family wealth. One variant of this approach is that while Yaakov should have generously given the soup to Esav, he did not take advantage of Esav since Esav, the great hunter, was not really on the verge of dying, was of sound mind at the time of the transaction and valued the soup as being equal to the value of having the birthright. How could this be? Ibn Ezra (on 25:34) argues that Yitzhak was poor and hence there was no expected gain from having the birthright. With this approach, the importance of the episode is not the inheritance itself, which is never mentioned again, but to show Esav’s character that because he was impulsive he was not worthy of Yitzhak’s blessing.
This approach also raises many questions. Was Yitzhak really poor? The Ramban (on 25:34) rejects the idea that Yitzhak was poor. If Yitzhak was poor, and the expected inheritance was truly a pittance, why did Yaakov want the birthright? Did Yaakov foresee Yitzhak becoming wealthy while Esav did not? If Esav really believed that the expected value of the inheritance was worthless, then how does his actions indicate that he was not worthy to receive the blessings? Also, could the inheritance have been so little that its expected value was equal to the value of the soup which Esav devoured in a few minutes? Furthermore, even if Esav was impulsive, is impulsiveness a reason to be disqualified from receiving the blessings of Yitzhak?
Another possibility within this second approach is that the birthright had no financial relevance because Esav was still legally the firstborn even after the sale, and then Yaakov did not take advantage of Esav since he did not gain anything tangible from the sale. Instead, the sale just signaled which brother had a greater connection to the family. Yaakov’s purchased the birthright to show his desire to be part of the family, especially to be closer with his father. (This would accord with the Or Hachayyim's suggestion, on 25:29, that Yaakov was cooking the soup to give to Yitzhak.) On the other hand, Esav was willing to sell the birthright for almost nothing since he did not value being part of the family, even though his father loved him greatly. A proof for this idea is the note in 25:34 that Esav despised the birthright, which would mean according to this approach that he despised his family. With this idea, the significance of the transaction was that the sale showed that Esav was not worthy of receiving Yitzhak's blessing since he did not want to part of the family, similar to his marriages outside the family, 26:34.
However, one might maintain that Esav really thought that he was on the verge of dying and he only despised the birthright after he sold it. This would be similar to the story of sour grapes, that when a person cannot get something then he disparages it. Also, later, he claimed that Yaakov tricked him out of the birthright, 27:36, which implies that he did not willfully relinquish the birthright, though maybe this statement was due to his later regret after having made the sale.
A third approach is that Yaakov sold the soup in order to rectify what he perceived as an injustice in the world. Yaakov was fighting the idea that the firstborn should receive greater honor or rights solely because he was born first, which in this case of twins, was just a few minutes. (Presumably, their "struggle" in Rivka's womb was to see who would come out first, and Yaakov was holding Esav's heel to symbolize his effort to be the firstborn, 25:22,26.) By selling the soup for the birthright, Yaakov was mocking the value of being firstborn, as he showed that it was worth just a bowl of soup.
A variation on this third approach is that Esav bragged about being the firstborn, and Yaakov though that this was doubly unfair. Thus, when Esav came home famished, Yaakov saw his chance to protest against this injustice and stop Esav from bragging.
Is there any evidence that Esav bragged about being the firstborn? 25:34 records that Esav called the soup the red, red, which is bizarre. Robert Altar (2004, p.131) writes, "although the Hebrew of the dialogues in the Bible reflect the same level of normative literary language as the surrounding narrative, here the text comes close to assigning substandard Hebrew to the rude Esav." Why did Esav not refer to the food as soup or red just once? What is the significance of the double reference to the color red?
In addition, 25:34 records that Esav was called Edom. The usual explanation of the name Edom is that it was due to this transaction that he used the double term red as part of the transaction. This understanding is based on the idea that everybody learned about this transaction. Yet, it seems that Yitzhak only learned of this transaction after he blessed Yaakov by mistake, see Rashi on 27:36.
My thought is that Esav was particularly fond of the color red, and his fondness for the color red in general caused others to call him Edom. This incident would then be one example of Esav always referring to the color red, and the word "therefore" in 25:30 is referring to his general pattern of referring to the color red and not specifically to the case here. (See Yehuda Keel, 2000, p.231, footnote 57, who quotes an interesting suggestion from Baal ha-Deyukim(?), that Esav called himself Edom unrelated to this sale.)
Independent of whether this incident is the source of Esav being called Edom, why did he have an affinity for the color red? The answer is that when he was born he was ruddy, 25:25, and the color red was his code that he was the firstborn. Here he referred to the soup twice as red, and Yaakov understood that he was bragging about being the firstborn and possibly, he was even demanding the soup because he was the firstborn. This ticked off Yaakov, and then, and only then, he came up with the idea of demanding that Esav sell him the firstborn, which meant that Esav could no longer brag about being the firstborn to Yaakov. According to this approach, Yaakov did take advantage of Esav by the sale, but it was to stop Esav from bragging about being the firstborn.
With either variation of the third approach, the importance of this story is that this is another example of the fight against the firstborn status in the book of Bereshit. This is the pattern in Bereshit, starting with Kayin and Hevel (chapter 4) and ending with Efrayim and Menashe (chapter 48), that the older brother is not intrinsically superior to the younger brother.