And also that nation, whom they shall serve, will I judge: and afterward shall they come out with great substance.

Wesley's Notes

15:14 That nation whom they shall serve, even the Egyptians, will I judge - This points at the plagues of Egypt, by which God not only constrained the Egyptians to release Israel, but punished them for all the hardships they had put upon them. The punishing of persecutors is the judging of them; it is a righteous thing with God, and a particular act of justice, to recompense tribulation to those that trouble his people.
3. The deliverance of Abram's seed out of Egypt. And afterwards shall they come out with great substance - Either after they have been afflicted 400 years, or, after the Egyptians are judged and plagued.

Matthew Henry's Concise Commentary

15:12-16 A deep sleep fell upon Abram; with this sleep a horror of great darkness fell upon him: a sudden change. The children of light do not always walk in the light. Several things were then foretold. 1. The suffering state of Abram's seed for a long time. They shall be strangers. The heirs of heaven are strangers on earth. They shall be servants; but Canaanites serve under a curse, the Hebrews under a blessing. They shall be suffers. Those that are blessed and beloved of God, are often sorely afflicted by wicked men. 2. The judgment of the enemies of Abram's seed. Though God may allow persecutors and oppressors to trample upon his people a great while, he will certainly reckon with them at last. 3. That great event, the deliverance of Abram's seed out of Egypt, is here foretold. 4. Their happy settlement in Canaan. They shall come hither again. The measure of sin fills gradually. Some people's measure of sin fills slowly. The knowledge of future events would seldom add to our comfort. In the most favoured families, and most happy lives, there are so many afflictions, that it is merciful in God to conceal what will befall us and ours.