Eisav selling the first born

Rav Daniel Dombrov

How could Eisav sell the rights of the first born to Yakov?

Regarding the double portion of inheritance, it is considered a davar shelo ba l’olam.
Regarding the leadership and other aspects, these are intangibles and considered a davar she’ayn bo mamesh.

Ketzos- this is determined based on an individual’s ability to renounce his rights of inheritance.

A husband may renounce his rights to inherit his wife because his ownership is only midrabannan. Therefore, a bechor, which is de’raysa, can not renounce his rights of inheritance.

Ran- Ordinarily, one cannot renounce his rights to inheritance after he is in a position to inherit. [A husband may renounce his rights only prior to marriage.] Nevertheless, the first born is unique, in that it is not considered to be a standard inheritance, but rather, a Torah mandated ‘gift’. If so, Eisav was in a position to at least renounce his right to the first born.

Ohr HaChaim – davar shelo ba l’olam is lacking gemiras daas. When one is in a desperate situation, there is gemiras daas even on a davar shelo ba l’olam. Eisav was acting out of desperation, which accords full gemiras daas to the kinyan.

Additionally, concerning selling an intangible item- davar she’ayn bo mamesh– when one accompanies the transaction with an oath- a shevuah- the transaction is binding.

Rosh- prior to Matan Torah, there was no issue transacting in a davar shelo ba l’olam. The Rivosh disagrees with this.