In reasoning in this way, Paul shows us the right way to handle difficulties in the word of God. The right approach is to establish a firm foundation based on what we know to be certain truth, and then to move on from here. What is more difficult must be thought through from this starting point. God never makes a mistake and all that he has done is true and well-ordered. When we come across what seems to be a difficulty, we are confident, knowing that the answer to it will reveal some wonderful further view of the truth which will add to our understanding. The solid ground is the truth that God cannot contradict himself, and therefore the covenant which he gave to Abraham cannot be undone by something else which he does later.
There is a wrong way of interpreting this verse which completely misses the point. It is to say that the reason the law, which came after the promise, does not make the promise of none effect is that the two are compatible with each other, that they offer salvation on the same terms, that Sinai was a gracious covenant and it is perfectly possible to reconcile law and gospel if only Sinai is understood in the right way. Some try to strengthen this case by saying that what Paul has in mind is the ceremonial law with its types and shadows which pointed to the redeeming work of Christ, not the moral law which offered salvation by works. They teach that this is the predominant element of Sinai, which was in fact an administration of the covenant of grace. And yes, the ceremonial law was gracious in character, though ineffective in itself because it only held out the promise of future blessing in Christ. But that does not change the essential character of Sinai which Paul has assumed throughout: that it was a covenant of works and offered salvation only on a legal basis. Paul sees the promise to Abraham and the law given on Sinai as in head-on collision with each other; this has been his argument all along. The reason why the law does not disannul the promise coming before it is that it was never intended to offer salvation. The law was not in competition with the promise because the law was intended to serve a completely different purpose and to leave the terms of salvation in the Abrahamic covenant completely unchanged. The truth is that the Abrahamic covenant and the law coexisted at the same time, but that any one individual was either under one or under the other, but never under both at the same time.
By what reckoning was the law given 430 years after the Abrahamic promises? Israel was in Egypt 430 years and the promises were given to Abraham about 215 years before this. The answer is probably that Paul avoids any accusation of exaggerating his case in lengthening the time between the giving of the promise and Sinai, and therefore takes the latest time at which the covenant was confirmed to Jacob just before Israel went into Egypt.