Why did HaSh-m find it necessary to assure Yitzchak that He would fulfill the Shevu'ah that He made to Avraham?
Ramban #2: He swore to Yitzchak that He would incorporate him in the Shevu'ah that He made to Avraham - i.e. that he would be a Berachah among the nations (that 'all the nations will be blessed through him'). 3
Ha'amek Davar (to 26:4): This is not the Shevu'ah of Bris Bein ha'Besarim to receive Eretz Kena'an, for HaSh-m already promised him the land in this verse! Rather, it is the Shevu'ah at the Akeidah, that Avraham's seed will be like the stars 4 and like the sand.
Therefore the Torah writes, "I will remember the covenant (that I made) with Yaakov, also the covenant with Yitzchak, and also the covenant with Avraham I shall remember" (Vayikra 26:42).
As HaSh-m promised Avraham (22:18). Indeed, HaSh-m made the same promise to Yaakov (28:14).
Ha'amek Davar (ibid.): Even though the next verse promises this, Avraham's Berachah was that they will illuminate the nations (with Chochmah). HaSh-m added to Yitzchak that they will be an influence of abundance and wealth.
Why did HaSh-m say to Yitzchak, "Shechon (settle) in the land that I will tell you..." (26:2), and immediately afterwards, "Gur (sojourn) in this land"?
Ramban (to 26:2): HaSh-m probably meant that Yitzchak should, from now on, live only in lands that HaSh-m designates, 1 and as for now, he should sojourn in Eretz Pelishtim.
Oznayim la'Torah (to 26:2): First HaSh-m told Yitzchak that as an Olah Temimah, he was not permitted to leave Eretz Kena'an to go anywhere else without express permission. Then, He told him that, as far as the current situation was concerned - to escape the famine, he was permitted to remain in Eretz Pelishtim.
Ohr ha'Chayim (to 26:2): HaSh-m initially said, "in the land that I will tell you" (26:2) so he will be rewarded for accepting whatever HaSh-m will tell him, even if it is against his desire.
Malbim (to 26: 2-3): Do not think that you must go to Egypt due to My decree. "Settle in the land that I will tell you... ," and since you follow My word, you are by yourself, 2 and Hashgachah Pratis will cling to you. The exile that you were obligated will be fulfilled in this land, by going from place to place.
Ha'amek Davar (to 26: 2-3): "Shechon" implies seeding and planting (Midrash Raba). From here, we learn the Mitzvah of settling Eretz Yisrael (Gitin 8b). 3 This was given to Yitzchak, and not to Avraham, for Yitzchak was the source of Yisrael dwelling in the land. Even so, you should feel like a Ger (stranger, in this world), like Divrei ha'Yamim I 29:15. Also, HaSh-m showed that at the beginning [Yisrael] will be under the Pelishtim, and will suffer much, until David.
Moshav Zekenim: We do not find that HaSh-m told him where to dwell, only 'la'Gur' (stay temporarily). We find that later, he fixed himself in Be'er Sheva. Only in his old age, he moved to Kiryas Arba.
I.e. you have nothing to do with the inhabitants of this land, on whom the famine was decreed.
The Gemara permits telling a non-Jew to write a document on Shabbos (which is normally forbidden) to buy land in Eretz Yisrael, for the sake of settling Eretz Yisrael. It does not bring a source. (PF)
Why did HaSh-m allow Avraham to go to Egypt, but He did not allow Yitzchak?
Ramban (to 26:1, citing Bereishis Rabah 64:3): Because Yitzchak was an Olah Temimah, and it was not fitting for him to go to Chutz la'Aretz. 1
Moshav Zekenim (to 26:2): If he would go to Egypt, he could not return before the deadline (400 years), for so HaSh-m decreed.
Malbim (to 26:1): Avraham went to Egypt because he was not yet worthy of the miracle to be blessed here (in Eretz Kena'an) during a famine - (a) he was not yet tested enough, and (b) the land was promised to him, but was not yet given to him, so it did not have the proper Kedushah. Yitzchak was already tested at the Akeidah, and Avraham had already acquired the land, so Yitzchak merited special Hashgachah here, and did not need to leave. 2
Malbim (to 26:3) - Yitzchak did not conquer the land, so we must say that HaSh-m's promise to give it to him was for Kedushah and Hashgachah Pratis. 3 Avraham went to Egypt before HaSh-m swore [to give the land to him].
Moshav Zekenim: Rashi (to 26:12) learns from "ba'Aretz ha'Hi" that Gerar is in Chutz la'Aretz! We find that it is not within the borders of Eretz Yisrael! Really, it was in the land, but not in the primary (middle) part. (Also Hadar Zekenim (to 26:8) says so.) Part of Eretz Pelishtim was in the land. (We find that Avraham was adamant that Yitzchak not go to Chutz la'Aretz (24:6). Had Avraham not told him the reason?! (PF)
Seforno: HaSh-m instructed Yitzchak to remain in Kena'an, in the area that He designated for him, and that He would bless him specifically there with success - grazing-grass and all. Through remaining in Eretz Kena'an, the land that HaSh-m had promised via Avraham to give to him and his children, he would be treated like a prince of G-d there, and would establish ownership of the land.
And as such, the famine would not affect him.
Are the exiles of Avraham and Yitzchak a sign for what will happen to their descendants?
Ramban: Avraham's exile to Egypt was symbolical of Galus Mitzrayim (refer to 12:10:2:1). Yitzchak's exile to Eretz Pelishtim symbolized Galus Bavel. Bearing in mind that he was not permitted to go to Charan, going to Pelishtim, where Avraham had gone on another occasion, 1 can be considered going into exile in the land of his fathers, which symbolizes Galus Bavel. 2
What are the Berachos, "v'Ehyeh Imcha" and "va'Avarecheka"?
Ha'amek Davar: "V'Ehyeh Imach" is to save you from the hatred of Avimelech and his nobles. "Va'Avaracheka," with abundant grain.
Malbim to (26:4): A Tzadik over whom there is Hashgachah Pratis, is a conduit through which Berachah comes to the land - "v'Nivrechu Vecha Kol Mishpechos ha'Adamah" (12:3). Now, only you are blessed in this famine. Later, (a) when you are a big nation, and (b) you will conquer Eretz Kena'an, you will be a conduit for Berachah to others - "v'Hisbarchu b'Zaracha Kol Goyei ha'Aretz" (26:4).
Why did HaSh-m say "Lecha ul'Zar'acha"?
Ohr ha'Chayim #1: You will have success in farming the land.
Ohr ha'Chayim #2: You should make a Chazakah (acquire through living there) in Eretz Pelishtim, in which Avraham did not make a Chazakah. This is why it says "Kol ha'Aratzos ha'El."
Ohr ha'Chayim #3: He told him that the promises to Avraham are only for you; Yishmael has no share in them.
Ha'amek Davar: You should show dearness of the land, and not leave it.