וְיַעֲקֹ֞ב נָתַ֣ן לְעֵשָׂ֗ו לֶ֚חֶם וּנְזִ֣יד עֲדָשִׁ֔ים וַיֹּ֣אכַל וַיֵּ֔שְׁתְּ וַיָּ֖קָם וַיֵּלַ֑ךְ וַיִּ֥בֶז עֵשָׂ֖ו אֶת־הַבְּכֹרָֽה׃ (ס)
Entonces Jacob dió á Esaú pan y del guisado de las lentejas; y él comió y bebió, y levantóse, y fuése. Así menospreció Esaú la primogenitura.
Rashi
ויבז עשו THUS ESAU DESPISED—Scripture testifies to his wickedness: that he despised the Service of the Omnipresent!
Chizkuni
ויעקב נתן לעשו וגו, meanwhile Yaakov had already given to Esau, etc.;” at the same time when Esau had paid him money for the birthright, Yaakov had already fed him bread and a dish of lentils as proof that the sale had taken place. This was a normal procedure when commercial transactions took place in those days. We find an additional example of this in Genesis 31,46, when Yaakov and Lavan conclude a peace treaty by the stone monument.
Da'at Zekenim
ויבז עשו את הבכורה, “Esau had already displayed disdain for the asset known as birthright.” This is why Yaakov bought it from him. We find that Rabbi Yehudah Hachassid, referring to this verse, claims that if one finds a confirmed sinner in the possession of holy texts such as a Torah scroll, it is permissible to use subterfuge to get him to sell it.