וַיֹּ֣אמֶר יַעֲקֹ֗ב הִשָּׁ֤בְעָה לִּי֙ כַּיּ֔וֹם וַיִּשָּׁבַ֖ע ל֑וֹ וַיִּמְכֹּ֥ר אֶת־בְּכֹרָת֖וֹ לְיַעֲקֹֽב׃
האָט יעקב געזאָגט: שװער מיר אַקאָרשט. האָט ער אים געשװאָרן, און ער האָט פֿאַרקױפֿט זײַן בכוֹרה צו יעקבן.
Or HaChaim
Alternatively, this can be understood as an oath. There are matters which are acquired only by means of an oath, such as items that are intangible. (the fragrance of the jasmin). Compare my commentary on Genesis 25,33 when Esau sold his birthright.