Question

Hello, Rabbi.
I saw a question about how Esau could transfer the birthright to Jacob, since the birthright is something that had not yet come into existence. At first I thought this was mere pilpul, but afterward I saw that the Tur discusses it in his commentary on the Torah in the name of the Rosh (Parashat Toledot), and even proves from it that by means of an oath, acquisition can take effect regarding something that has not yet come into existence. This difficulty is also discussed in the responsa of the Rivash (sec. 328), in Ketzot HaChoshen (sec. 278), and in the responsa Shoel U’Meshiv (third edition, vol. 2, sec. 163). And I do not understand what possible basis there is for discussing the laws of acquisitions based on a story in the Book of Genesis. What do you think?