John Gill Commentary Genesis 25:33

"And Jacob said, Swear to me first. And he sware unto him. And he sold his birthright unto Jacob." — Genesis 25:33 (ASV)

And Jacob said, swear to me this day For the more sure and certain confirmation of the bargain; and by this oath oblige himself to let him peaceably enjoy the birthright, nor seek to revoke it, or dispute it with him, or disturb him in the possession of it.

and he swore to him; that he would abide by the bargain, and never give him any trouble on that account; and hereby he made it over to Jacob as firm as it could be; God himself being appealed to as a witness of it, whose will it was that Jacob should have the birthright, the blessing, and the promises.

and he sold his birthright to Jacob; with all the privileges and appurtenances of it, and that for one morsel of meat, as in (Hebrews 12:16).