Or HaChaim on Genesis 27:19/אור החיים על בראשית כ״ז:י״ט

1 אנכי עשו בכרך. "I am Esau your firstborn." Jacob meant that seeing he had purchased the birthright from Esau, he was now the legal Esau. He added: עשיתי כאשר דברת אלי, "I have done in accordance with what you have said to me," meaning that the reason you told Esau to hunt game, etc., was because you assumed that he was your firstborn.