Genesis 25:31

וַ יֹּאמֶר יַעֲקֹב מִכְרָה כַ יֹּום אֶת בְּכֹרָתְךָ לִי

And Heel is saying, `Sell her as the Hot-one^* אֶת-your firstborn^* to-myself.`

^* Strong’s #1062, bekorah. This is the feminine of bakar, first-born or firstling. The abstract idea of “birthright” is conjecture. James Strong admits “abstractly primogeniture - birthright”. It us used concretely in Genesis 4:4 “firstborns of his sheep” and also Deuteronomy 12:6, 12:17, 14:23, and 21:17.