If a person said, "It is forbidden to me as a vow that I derive no benefit from the seed of Abraham," he is forbidden to benefit from Israel but permitted to benefit from the nations of the world. But is there not Ishmael, who is also of Abraham's seed? The verse states, "For through Isaac shall offspring be called to you" (Genesis 21:12), so Ishmael is excluded. But is there not Esau, who descends from Isaac? Scripture says "through Isaac," meaning through some of Isaac, but not all of Isaac, so Esau too is excluded.
A Vow Against Abraham's Seed and the Limits of His Line
Yalkut Shimoni on Torah 94:6
אָמַר קוֹנָם שְׁאֵינִי נֶהֱנֶה לְזֶרַע אַבְרָהָם אָסוּר בְּיִשְׂרָאֵל וּמוּתָר בְּאוּמוֹת הָעוֹלָם, וְהָאִיכָּא יִשְׁמָעֵאל כִּי בְּיִצְחָק יִקָּרֵא לְךָ זָרַע כְּתִיב וְהָאִיכָּא עֵשָׂו בְּיִצְחָק וְלֹא כָּל יִצְחָק.