A Covenant Ratified
I see this "one man's covenant" sort of like a will, it is not set aside and nothing is added to it.
In Paul’s example, the covenant refers to the promise that God made to Abraham (before God instructed Abraham to be circumcised; Genesis 17:26). This covenant was ratified not by circumcision, but by God himself (Genesis 15:9–21).
Galatians 3:15New American Standard Bible: 1995 Update
Brethren, I speak in terms of human relations: even though it is only a man’s covenant, yet when it has been ratified, no one sets it aside or adds conditions to it.