Commentaire sur La Genèse 15:13

כי גר יהיה זרעך THAT THY SEED SHALL BE A STRANGER — From the birth of Isaac until Israel left Egypt was a period of 400 years. How so? Isaac was 60 years old when Jacob was born, and Jacob when he went down to Egypt himself stated, (Genesis 47:9) “The days of the years of my sojournings are a hundred and thirty years”, making together 190 years. In Egypt they were 210 years — corresponding to the numerical value of the word רדו (see Rashi in Genesis 42:2) — making altogether 400 years. If, however, you say that they were in Egypt 400 years — well, Kohath was one of those who went down to Egypt with Jacob; go and add up the years of Kohath (130), those of Amram (137), and the 80 years that Moses was old when Israel left Egypt, and you only have about 350, and you really have to deduct all the years that Kohath lived after Amram was born, and those that Amram lived after the birth of Moses (Megillah 9a).

THAT THY SEED SHALL BE A STRANGER. This is a verse that is to be transposed, its purport being that “thy seed shall be a stranger for four hundred years in a land that is not theirs, and they shall enslave them, and they shall afflict them.” He has thus not specified the length of the period of servitude and affliction.
There are many cases in Scripture where verses must be transposed if they are to be interpreted properly. Thus: There came unto me the Hebrew servant, who thou hast brought unto us, to mock me;²⁸¹Further, 39:17. The meaning being: “There came unto me to mock me the Hebrew servant.…” And all countries came into Egypt to buy corn to Joseph;²⁸²Ibid., 41:57. The meaning being: “And all countries came into Egypt to Joseph.…” For whosoever eateth leavened bread, that soul shall be cut off from Israel, from the first day until the seventh day;²⁸³Exodus 12:15. The meaning being: “For whosoever eateth leavened bread, from the first day until the seventh day, that soul.…” In that day a man shall cast away his idols of silver, and his idols of gold, which they made for themselves to worship, to the moles and to the bats;²⁸⁴Isaiah 2:20. The meaning being: “In that day a man shall cast away to the moles and to the bats his idols.…” Come, and hearken, and I will declare, all ye that fear G-d, what He hath done for my soul;²⁸⁵Psalms 66:16. The meaning being: “All ye that fear G-d, come.…” They cry unto Me, My G-d we Israel know Thee;²⁸⁶Hosea 8:2. The meaning being: “Unto Me crieth Israel, ‘My G-d, we know Thee.’” And they shall be Mine, saith the Eternal of hosts, in the day that I do make, even Mine own treasure, and I will spare them.²⁸⁷Malachi 3:17. The meaning being: “And they shall be Mine treasure, saith the Eternal.…” There are many other such verses.
The sense of the verse is: “Even though I tell you that I have given this land to your children, you should surely know that before I give it to them they shall be strangers for four hundred years in a land not belonging to them, and they shall also enslave them and afflict them.”
Rabbi Abraham ibn Ezra said [that the verse should be interpreted as follows]: “You should surely know that your children shall be strangers in servitude and affliction until the end of a four hundred year period commencing from this day of the covenant.” If so, G-d informed Abraham of the time of the redemption, but He did not inform him of the exact length of the exile. This too is correct.

Know for sure. God’s main point here was that the inheritance of the Land had to wait until the sin of the Emorites was complete (v. 16). Along the way, however, He revealed the suffering that would befall some of Avraham’s offspring when they failed to live up to their obligations.