Tafsiran Kejadian 25:33: Chizkuni, Ramban, Sforno & Or HaChaim

השבעה לי כיום “swear it to me as of this day. Yaakov requested an oath that Esau would not change his mind about this deal at some time in the future. This was the only way that the sale would have meaning for him. He did so because the Talmud teaches that if someone announces: “I will sell you whatever I will inherit from my father,” such a statement is legally meaningless. (Baba Metzia 16).

SWEAR TO ME THIS DAY. When Esau said, “What is this birthright to me? I do not desire it,” Jacob said to him, “Swear to me that you will not desire it, nor will you inherit it forever.” Thereupon he swore to him, and following that he sold it to him, and Jacob gave him the purchase price⁶⁴That is, the money. This is in accordance with the opinion stated above. Although Ramban does not agree with it, he nevertheless mentions it as an alternate interpretation of the verse. or the pottage he desired.
It is possible that Esau said, “What is this birthright to me? It is sold to you,” and Jacob said, “Swear to me that you will never complain about the sale.” And when Scripture says that first he swore to him and then he sold it to him, its intent is as if it said that he sold it to him first and then swore to him.

השבעה לי, seeing that the purchase under discussion was something abstract, the oath would replace the act of physically taking possession of the object purchased and moving it.