וַיֹּ֣אמֶר לְאַבְרָ֗ם יָדֹ֨עַ תֵּדַ֜ע כִּי־גֵ֣ר ׀ יִהְיֶ֣ה זַרְעֲךָ֗ בְּאֶ֙רֶץ֙ לֹ֣א לָהֶ֔ם וַעֲבָד֖וּם וְעִנּ֣וּ אֹתָ֑ם אַרְבַּ֥ע מֵא֖וֹת שָׁנָֽה׃
And He said to Abram, “Know well that your offspring shall be strangers in a land not theirs, and they shall be enslaved and oppressed four hundred years;
We can infer from this verse that the words "A land not theirs" would seemingly be extra, as it would have been suffice for the Torah to just say 'that your children will be strangers [amongst the nations].;' without mentioning in a land not their own, we already know once you are a stranger you are not in your own land!
Furthermore we have a wondrous question here. In regards to the destruction of Sedom and Gemora, we see that Abraham prayed greatly to HaShem for the wicked and evil people of those lands to be saved from destruction.
As it states in the verse -
Abraham came forward and said, “Will You sweep away the innocent along with the guilty?
Yet, over here when HaShem tells Abraham that his children will experience exile, and will be strangers in a land not their own, Abraham does not speak up and pray on the children of Israels behalf to cancel the decree of the exile upon them. Why does he pray for the evil men of Sedom, but not his own grandchildren!?
With the help of Heaven we will suggest an answer based on the words of the Holy Arizal, may his memory be of life in the world to come. As he says - The reason that the people of Israel get exiled from exile to exile, has nothing to do with the fact that they are evil people and sinners, G-D forbid, rather, the purpose of the exile of Israel is to extend the mission of Abraham, may he be loved by Heaven, which is [the mission is] to publicize the reality and blessedness of HaShem in this world, and in a set and due time when the mission is completed and finalized, only then will we leave the reality of exile.
Similarly this is taught by the Holy Rabbi of Ruzin, may his memory be of the world to come, on his commentary on a well known verse from the Musaf prayer on the Festival Holidays.
וּמִפְּנֵי חֲטָאֵינוּ גָּלִינוּ מֵאַרְצֵנוּ..
"And because of our sins we were exiled from our lands.."
"The explanation here is rereading that words that when it says 'Because of', which in Hebrew is 'U'Mipnei', to rather be read differently. That same word can be read as a root for the world 'Lifnei' (same letters), meaning prior to, or beforehand, so the verse is not saying 'Because of our sins', rather it is saying 'Before our sins'. Meaning to say, that prior to our sins ever happening, it was already decreed on the people of Israel the reality of exile, and hence we were exiled from our land. And now that we find ourselves here in exile, we should know it's just to finalize the mission of Abraham our father, loved by Heaven. This mission is to publicize the name of the Blessed One throughout the world", and these are the words of the Rabbi of Ruzin.
And behold, as is the way of the world when two nations war with each other, the victor will then place different forms of taxes on the defeated nation, but not to kick out and dispel the nation that was defeated. However, this is not the case with Israel, in which case the victors simply kicked them out from their place of dwelling... And from the initial exile until now, we are still being evicted from nation to nation. The reason for this is because it is upon us to publicize our Blessed G-D, therefor its upon us to wander and travel the whole world.
This is why HaShem tells Abraham that "his children will be foreigners in a land not their own" (our initial question) - the explanation to our initial question rests in rereading the verse above instead of it saying that '“Know well that your offspring shall be strangers in a land not theirs' - it is 'Lo Lahem', (instead of saying not theirs, we can translate it as) 'Not on them' [not because of their sins], specifically because their job is to finish the mission placed on the shoulders of Avraham. Therefor they are exiled in order to continue this holy mission of Abraham, which needs to happen in a land not their own.
So that verse can be reread as '“Know well that your offspring shall be strangers, and it's not on them (not their fault)."
This explanation perfectly answers our initial question of wonder regarding why Abraham didn't question and petition HaShem on the need to exile his children, and why he didn't pray to cancel the decree, as he heard direct from the mouth of HaShem that the purpose of the people of Israel would be to continue his mission of publicizing the name of HaShem throughout the whole world, and therefor, Abrahams logic agreed with this reality, as Abrahams personal hope, salvation and desire was simply and only to publicize the name of Heaven to the world.