Translations
The promises were spoken to Abraham and to his Offspring. It doesn't say "offsprings," as if referring to many, but referring to one: "and to your Offspring", who is the Messiah.
Now the promises were spoken to Avraham and to his Seed. He does not say "and to seeds," as referring to many, but referring to one: "and to your Seed", who is the Mashiach.
Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
The promises were spoken to Abraham and to his seed. The Scripture does not say, “and to seeds,” meaning many, but “and to your seed,” meaning One, who is Christ.
Footnotes
Paul builds a midrashic argument on the singular form of σπέρμα (sperma, "seed/offspring"). In both Hebrew and Greek, collective nouns for "seed" are naturally singular even when referring to many descendants — standard usage. Paul, reading in the manner of Second Temple Jewish exegesis, treats the singular as a textual clue pointing to a specific individual: the Messiah. This is not grammatical sleight of hand but a recognised hermeneutical move, locating in the collective singular a hidden typological pointer embedded in the text itself.