Parashat Toledot tells the perplexing story of Yaakov’s “purchase” of the bekhora (“birthright”) from his older twin, Esav. In relating this incident, the Torah does not clarify what exactly the birthright entailed, which rights or privileges Yaakov acquired that day from his brother. Rashi (25:31), based on the Midrash (Bereishit Rabba 63:13), explains that Yaakov sought to acquire the rights to offer sacrifices. Back then, the privilege of performing the avoda(sacrificial rites) was granted to the firstborn, and thus Esav was naturally assigned to this position of “priesthood.” Yaakov correctly deemed Esav unworthy of this role, and thus jumped at the opportunity presented to usurp the birthright from his undeserving brother.
A number of later writers raised the question of how this kind of stature can be subject to purchase. The privilege of the birthright seems to have been the pre-Matan Torah equivalent of the kehuna. Clearly, a kohen could never “sell” his priestly privileges to a non-kohen. On what basis, then, was Yaakov able to “purchase” the status of birthright from Esav?
The Taz, in his Divrei David, answers that this status of bekhora did not, in fact, precisely correspond with the later institution of the kehuna. There was (quite obviously) no halakha in place banning non-bekhorim from offering sacrifices. Rather, the custom evolved to reserve this privilege for the firstborn. What Yaakov obtained from Esav, then, was not his personal status, but rather a promise to permanently forego his avoda rights and allow Yaakov to enjoy this privilege. This explains why the “transaction” concludes with Esav making an oath to Yaakov (25:33). An oath is precisely what Yaakov wanted; what he purchased was not any kind of asset, but rather Esav’s guarantee that he would forever more allow Yaakov the privilege of the avoda. (The Maharal, in his Gur Aryeh, explains similarly.)
It should be noted that the Gemara appears to have understood this “sale” differently. In Masekhet Sota (13a), the Gemara tells that before Yaakov’s burial in Me’arat Ha-makhpela, Esav arrived and protested his deceased brother’s right to the remaining plot. He insisted that he, not Yaakov, was entitled to the coveted burial site in the Makhpela cave. The Gemara records the exchange that ensued between Esav and Yaakov’s children, in which clear reference is made to the sale of the bekhora. In the end, it is indeed confirmed that the sale of the bekhora included the rights to burial in Makhpela, and even from the outset, Esav acknowledged that through the sale he had forfeited his right to a double portion of his father’s inheritance (see Rashi s.v. nehi di-zvinti). Clearly, then, the Gemara assumed that this transaction referred to the financial privileges of the bekhora, and not to the rights of the avoda. This is, indeed, the approach taken by several commentators, including the Rashbam and Ibn Ezra.