Rashi on Genesis 25:31/רש"י על בראשית כ״ה:ל״א

1מכרה כיום SELL ME THIS DAY (literally, like the day) — Explain it as the Targum renders it “as this day”: just as this day is certain, so make me a sure sale.

2בכרתך THY BIRTHRIGHT — Because the sacrificial service was then carried out by the first-born sons, Jacob said, “This wicked man is unworthy to sacrifice to the Holy One, blessed be He” (Genesis Rabbah 63:13).