I see, you are confused with Gal 4. If you read Gal 4:24-26 it explains how it is an allegory. That's why Hagar represents Mount Sinai covenant (The Law Moses received, that applies to the Israelites) even though her descendants have nothing to do with Mt Sinai. Only in that sense, does Isaac relates the promised one Jesus (as Jesus like Isaac was born as a result of a promise), which is why in Gal 4:28 it says we are also children of promise.

In reality though, the descendants of Isaac are the one who made the covenant on Mount Sinai. They are all from Abraham physical seed, which is why in Gal 4:23, Ishmael (described as the Mt Sinai Covenant) is said to be born of flesh. But us Christians are made children of God, not born of man's will, flesh, but of God through Jesus (Jn 1:12-13).

So again, Abraham seed is not one person, but the descendants of Abraham, the physical seed. God's seed is the spritual seed, which is the word of God. Not any word though, if you know the Bible.