Brothers, I am speaking according to a human perspective. Nevertheless, [when] the covenant of a man has been ratified, no one declares [it] invalid or adds additional provisions [to it].
References for Galatians 3:15
Now to Abraham and to his descendant the promises were spoken. It does not say, "and to descendants," as concerning many, but as concerning one, "and to your descendant," who is Christ.
References for Galatians 3:16
Now I am saying this: the law, that came after four hundred and thirty years, does not revoke a covenant previously ratified by God, in order to nullify the promise.
For if the inheritance [is] from the law, [it is] no longer from the promise, but God graciously gave [it] to Abraham through the promise.