Friends, let me give you an example from everyday affairs of the free life I am talking about. Once a person's will has been ratified, no one else can annul it or add to it.
Now, the promises were made to Abraham and to his descendant. You will observe that Scripture, in the careful language of a legal document, does not say "to descendants," referring to everybody in general, but "to your descendant" (the noun, note, is singular), referring to Christ.
This is the way I interpret this: A will, earlier ratified by God, is not annulled by an addendum attached 430 years later, thereby negating the promise of the will.
No, this addendum, with its instructions and regulations, has nothing to do with the promised inheritance in the will.