“And He brought us to this place, and He gave us this Land, a Land flowing with milk and honey.” (Devorim 26, 9)
The simple Peshat of this possuk is, that this is what a person says when he brought his first grown fruits of the year called bikkurim, to the Bais Hamikdash. One would thank Hashem for two things; firstly, he thanks Hashem that he was able to come to the Bais Hamikdash, and secondly, that he merited to live in Eretz Yisroel.
The Medrash Yalkut Shimoni, however, explains this possuk differently; the first phrase of this possuk, "vai'yih'vee’ai’nu ell hamakom hazeh” - “And You, Hashem, brought us to this place”- is referring to the Bais Hamikdash; and is the reason for the second phrase, "va'yeetain lanu ess ha'aretz ha'zos"- “and Hashem gave us this land”. In other words, the person who brings bikkurim- the first fruits- isn't thanking Hashem for living in Eretz Yisroel and being able to come to the Bais Hamikdash, two separate things, but he is thanking Hashem for giving us Eretz Yisroel in the merit of our coming to the Bais Hamikdash.
We have to understand what bothered the Medrash to reject the simple meaning of the possuk and to explain that the possuk is teaching us that in the merit of the Bais HaMikdash we received Eretz Yisroel.
Zera Shimshon explains that the Medrash was bothered with the order of the possuk. If the possuk is speaking about the person thanking for two separate things, it should have been written, "va'yeetain lanu ess ha'aretz ha'zos"- “and Hashem gave us this land”, which happened first, and then "vai'yih'vee’ai’nu ell hamakom hazeh”-“And You, Hashem, brought us to this place”- since we came to the land before we came to the Bais Hamikdash. Since it is not written like that but the phrase "vai'yih'vee’ai’nu ell hamakom hazeh”-“And You, Hashem, brought us to this place” is written before "va'yeetain lanu ess ha'aretz ha'zos"- “and Hashem gave us this land”. Therefore, it cannot be that he is thanking Hashem for two separate events but rather he is thanking Hashem for being able to live in Eretz Yisroel in the merit of coming to the Bais Hamikdash.
Zera Shimshon asks, however, what does it mean that Hashem gave Klal Yisroel Eretz Yisroel in the merit of coming to the Bais Hamikdash? Eretz Yisroel was given to the Avos, Avraham, Yitzchak and Yaakov many years earlier, way before Klal Yisroel came to the Bais Hamikdash! How then can the Medrash say that we received Eretz Yisroel in the merit of coming to the Bais Hamikdash when it didn't yet happen?
Zera Shimshon answers that Hashem doesn't want the Bais Hamikdash to be given to Klal Yisroel as a gift, but Hashem wants the Bais Hamikdash to be acquired.
We see this idea from the way Dovid HaMelech bought Har HaBayis, the Temple Mount, to be used as the place of the Bais Hamikdash. Dovid looked for a place to bring a sacrifice at the time of a terrible plague, and Aravna, a Non-Jew who owned the field which was eventually the place of the Bais Hamikdash, offered to give it to Dovid for free. Dovid refused the offer and said to him, "No. I will only buy it from you at a price; so that I will not offer to Hashem burnt-offerings [which I had received] for nothing." (Shmuel Bais 24/24)
The Gemara (Zevachim 116b) explains that Dovid didn't mean that he would personally pay for Aravna's field with his own money, Dovid collected fifty shekel from each tribe, a total of six hundred shekel in order for all of Klal Yisroel to take part in the purchase.
We learn from here the above mentioned idea; the place of the Bais Hamikdash should be acquired by Klal Yisroel, and not given by someone as a present.
According to this, if Hashem would have fully given the Land to the Avos then Har HaBayis, which is situated in the middle of Eretz Yisroel, would have certainly also been included in the gift and there would be no reason to buy the land. Since Hashem wanted Klal Yisroel to buy Har HaBayis; it must be that even though it seems like the Land was a gift to the Avos, Hashem didn't fully give it to them. The Medrash teaches us that Eretz Yisroel became fully Klal Yisroel's, only after Dovid and the rest of Klal Yisroel bought Har HaBayis, and built the Bais HaMikdash. In the merit of this, Hashem finalized the gift of Eretz Yisroel.
Zera Shimshon adds that there is an allusion to this in Sefer Bereishis. Immediately after Hashem informed Avraham Avinu, "l'za'arecha ehtain ess ha'aretz hazeh"-To your offspring’s I will give this Land, it is written that Avraham built a mizbayach. Rashi explains the reason for this was to thank Hashem that he will inherit Eretz Yisroel. However, according to the Medrash in which we opened with, it is coming to allude to the fact that the inheritance of Eretz Yisroel will be in the merit of the Bais Hamikdash!
