Galatians 3:17
16 Now the promises were spoken to Abraham and to his descendant. Scripture does not say, “and to the descendants,” referring to many, but “and to your descendant,” referring to one, who is Christ. 17 What I am saying is this: The law that came 430 years later does not cancel a covenant previously ratified by God, so as to invalidate the promise. 18 For if the inheritance is based on the law, it is no longer based on the promise, but God graciously gave it to Abraham through the promise.
Acts 7:6
5 He did not give any of it to him for an inheritance, not even a foot of ground, yet God promised to give it to him as his possession, and to his descendants after him, even though Abraham as yet had no child. 6 But God spoke as follows: ‘Your descendants will be foreigners in a foreign country, whose citizens will enslave them and mistreat them for 400 years. 7 But I will punish the nation they serve as slaves,’ said God, ‘and after these things they will come out of there and worship me in this place.’
Notes and References
"... A chronological problem exists in Acts 7:6, where Stephen said - Israel would be enslaved and mistreated 400 years. For in Galatians 3:17 Paul implied that the period of time from the Abrahamic promise in Genesis 15:13-16 to Mount Sinai was 430 years. The difference between the 400 and 430 years can easily be accounted for by understanding that Stephen used round numbers. Another explanation is that the 400 years was the actual time of bondage whereas the 430 years described the time from the confirming of the covenant in Genesis 35:9-15 to the Exodus, which occurred in 1446 B.C. The main problem, however, is the time Israel spent in bondage in Egypt. If Galatians 3:17 means it was 430 years from the· promise given to Abraham (Genesis 15) to the Exodus, the time in Egypt would then be 215 years. However, if Acts 7:6 is taken at face value the bondage was 400 years. Perhaps the best solution is to say Paul was looking at periods of time. ..."
Walvoord, John F. The Bible Knowledge Commentary: An Exposition of the Scriptures (p. 370) Victor Brooks, 1983