Abraham was was made promised even before the law was enacted. And that promise was to him and his offspring.
“16Now the promises were made to Abraham and to his offspring. It does not say, ‘And to offsprings,’ referring to many, but referring to one, ‘And to your offspring,’ who is Christ. 17This is what I mean: the law, which came 430 years afterward, does not annul a covenant previously ratified by God, so as to make the promise void;” – Galatians 3:16-17 (ESV)
Promises of God are forever and cannot be changed. The covenant that was with Abraham remained in place until Jesus Christ. And even then, the promise of Jesus Christ was in place, so the promise never changed or was added to or subtracted from.
“21Is the law then contrary to the promises of God? Certainly not! For if a law had been given that could give life, then righteousness would indeed be by the law. 22But the Scripture imprisoned everything under sin, so that the promise by faith in Jesus Christ might be given to those who believe.” – Galatians 3:21-22 (ESV)
Promises made by God are unchanged. They cannot be amended. God knows and has known all for all time and when making the promises He has the now and later in mind. I have to believe the promises of God and not hang on the law to get me there. I am a child of God and therefore I believe in the promises of God.
“28There is neither Jew not Greek, there is neither slave nor free, there is no make and female, for you are all one in Christ Jesus. 29And if you are Christ’s, then you are Abraham’s offspring, heirs according to promise.” – Galatians 3:28-29 (ESV)
What about you, are you relying on the law? Are you aware of the unchanging promise of God? Do you believe in the promise?