Parashas Toldos · Passuk 25:33
But Yaakov said, “Swear to me as this day.” So he swore to him, and sold his birthright to Yaakov.
Classical on the oath of sale (classical source on this passuk; not listed as a numbered Torah Temimah unit here).
Lemma / focus: השבעה לי כיום.
English: Swear to me a complete sale as this day; he swore to him and sold his birthright.
Derash on the verse (plain sense with classical emphasis) — not a formal named middah of the 13: the oath makes the sale of the birthright final.
Yaakov required an oath that very day; Esav swore and sold the birthright, sealing a transfer that could not be undone by later denial.