Charles Ellicott Commentary Genesis 15:14

"and also that nation, whom they shall serve, will I judge: and afterward shall they come out with great substance." — Genesis 15:14 (ASV)

That nation. —Had it been expressly revealed that the country that would afflict them was Egypt, the patriarchs might have been unwilling to go there; but the reference to the plagues in the denunciation of judgment, and to the spoiling of the Egyptians in the promise that they should come out with great substance (Exodus 12:36), provided sufficient detail for future guidance and for their assurance in the future that the promise had been fulfilled.