Brothers and sisters, let us think in human terms: Even an agreement made between two persons is firm. After that agreement is accepted by both people, no one can stop it or add anything to it.
God made promises both to Abraham and to his descendant. God did not say, "and to your descendants." That would mean many people. But God said, "and to your descendant." That means only one person; that person is Christ.
This is what I mean: God had an agreement with Abraham and promised to keep it. The law, which came four hundred thirty years later, cannot change that agreement and so destroy God's promise to Abraham.
If the law could give us Abraham's blessing, then the promise would not be necessary. But that is not possible, because God freely gave his blessings to Abraham through the promise he had made.