'Contract' in the Bible

Brothers and sisters, I offer an example from everyday life: When a covenant has been ratified, even though it is only a human contract, no one can set it aside or add anything to it.

Now the mediator or go-between [in a transaction] is not [needed] for just one party; whereas God is only one [and was the only One giving the promise to Abraham, but the Law was a contract between two, God and Israel; its validity depended on both].