Among his many travails throughout Eretz Kena’an and its environs, Avram’s trip to Mitzrayim turns out to be one of the most significant in his life. He displays a true faith in Hashem, survives a potentially fatal encounter with Par’oh, and returns with a significant amount of money. When Avram comes back to Eretz Kena’an with his cousin Lot, they try to live together, but quickly find it to be impossible. Avram therefore gives Lot a choice: he could go in any direction he chooses, and Avram would go the opposite way. Immediately after this incident, Hashem tells Avram, “… Ki Et Kol HaAretz Asher Atah Ro’eh Lecha Etnenah ULeZar’acha Ad Olam" " … Because all the land that you see, I will give it to you and to your descendants forever" (BeReishit 13:15). The context of this promise seems odd; why would Avram be promised the entire land right after he offered Lot some land?
Rav Lord Jonathan Sacks poses an answer. When we look at the obligatory gifts of Bnei Yisrael, we gain the perspective that God owns everything, and when we give something of ours it shows that we understand and acknowledge His ownership. When a man follows the laws of Shemitah or gives Terumah it is clear that he knows that God is in charge and man is just watching His things. When we obtain items such as animals or produce, the first of those must be returned to God. He created them, and without Him they would not exist; by giving something back we acknowledge this. When Avram demonstrated that he understood this tenet of monotheism by offering part of Eretz Yisrael to someone else, he was given the entire land as a reward.
Rav Shimon Schwab, in his book on prayer, quotes a Gemara from Gittin 57a. The Gemara states that if one, during Keriat Shema, intends to give up his life, it is like he has done it. At the point that one forgoes a piece of his own, he is on a higher level. In the case of Terumah, only after the portion has been taken can the produce be brought inside the house. When a person offers his life for Kiddush HaSheim, according to Chazal, he is transformed from the status of a Mekabeil, a receiver, to a Mashpia, a giver. Avram became worthy of having the land and a people for all generations when he let Lot choose some land for his own.
Simply put, in the words of Chief Rabbi Sacks, "We only own what we are willing to share."