וַיֹּ֣אמֶר יַעֲקֹ֗ב הִשָּׁ֤בְעָה לִּי֙ כַּיּ֔וֹם וַיִּשָּׁבַ֖ע ל֑וֹ וַיִּמְכֹּ֥ר אֶת־בְּכֹרָת֖וֹ לְיַעֲקֹֽב׃
Jacob dit: "Jure le moi dès à présent." Et il lui fit serment et il vendit son droit d’aînesse à Jacob.
Derashot HaRan
And Jacob saw that the selling of the birthright would be to Esau's shame and humiliation. He, therefore, said (Genesis 25:32): "Swear to me as this day." His intent was not that this bind Esau for fear of his soul. For one such as he would not be bound by a vow or any manner of oath. (Our sages have already said [Bava Bathra 16b] that Esau had perpetrated five transgressions on that day, one of which was denial of the Holy One.) But because he foresaw the possibility that the sale might be contested (the birthright accorded by such a father as Isaac not being barterable for money), he desired to confirm the sale through Esau's oath, so that no further litigation be necessary if Esau contested the transaction. (To this should be added the fact that it is the practice of the peoples to this very day to secure those transactions which have some inherent weakness by means of an oath on the part of the leaser or seller.)