Brothers and sisters, let me use an example from everyday life. No one can cancel a person's will or add conditions to it once that will is put into effect.
The promises were spoken to Abraham and to his descendant. Scripture doesn't say, "descendants," referring to many, but "your descendant," referring to one. That descendant is Christ.
This is what I mean: The laws [given to Moses] 430 years after God had already put his promise [to Abraham] into effect didn't cancel the promise [to Abraham].
If we have to gain the inheritance by following those laws, then it no longer comes to us because of the promise. However, God freely gave the inheritance to Abraham through a promise.