"He believed in Hashem." The Midrash derives that we were redeemed in the merit of Emunah. Why is this so?
Maharal (Netzach Yisrael, Ch. 29, p. 139): Exile is separation from Hashem, whereas Emunah is attachment to Hashem with all of one's being. Therefore, Emunah is the key to redemption.
Who considered what to be Tzedakah, on whose part?
Ramban: It was Avraham who considered Hashem's promise a righteous one 3
Because he did not query Him (Targum Yonasan). See also Na'ar Yonasan.
Rosh: This shows that complete Emunah is as good as monetary Tzedakah or Tzedakah activities, regarding all matters that Hashem has told us depend upon Tzedakah (e.g. the Redemption of Tziyon, Yeshayah 1:27).
Ramban asks (on Rashi's explanation): If, besides being a Navi, Avraham would later believe Hashem sufficiently to slaughter his beloved son, why did Hashem praise him here merely for believing that he would have a son?
"And [Hashem] considered it righteousness for him." Why is it so praiseworthy that a prophet would believe his prophecy?
Maharal (Gevuros Hashem, Ch. 7, p. 41): Hashem's assurance regarding children could have been held back due to even a small sin, (for natural matters are on a low level in Creation, and are easily liable to change). Hashem brought Avraham "outside," to a higher level, and changed his name, so that even Mazal could not affect him. This was an astounding promise, and Avraham accepted it.
QUESTIONS ON RASHI
Rashi writes: "[Avraham] believed in Hashem - [in that] he did not ask for a sign regarding this." But why not interpret it as referring to the belief itself in Hashem's promise?
Gur Aryeh: It is obvious that a prophet would believe the word of Hashem that he had been told; rather, Avraham is praised for not asking for a sign.
Rashi writes: "He did not ask for a sign." Why is Avraham praiseworthy for this; surely one who truly believes in Hashem does not need a sign?
Gur Aryeh: It is possible that even after Hashem makes an assurance, sin on the part of the intended recipient would impede its fulfillment. It would be possible to ask for a sign that this would not happen; 1 although this would constitute a slight lack of faith. Avraham did not ask for such a sign.
Rashi writes: "He did not ask for a sign." Why then did Avraham ask for a sign regarding inheriting the Land (15:8)?
Ramban: Avraham asked for an assurance that even if the Kena'anim would repent, Hashem would not relent concerning their land. 1
Mizrachi: Hashem initially made the promise regarding the Land in verse 12:7, before the war of Chapter 14. It is mentioned here only by the way (15:7), "I [initially] brought you out of Ur Kasdim to give you this Land." Avraham feared that his merit may have been diminished by the war, and he therefore asked for a sign that he would still inherit it. The promise of children, on the other hand, was made now, after all of the miracles, and therefore Avraham did not fear that it would not be fulfilled. 2
Gur Aryeh: When an assurance of a gift is made without a sign, there is room to worry that the assurance is conditional, and can be impeded by the recipient's conduct. After all, the gift has not been given yet. Hashem's promise of children, however, was not a promise of a future gift, but a change in Mazal effective immediately. Avraham was not afraid that sin would impede this, for the blessing of children in general is dependent upon Mazal and not on merits (Moed Katan 28a).
Mizrachi asks on Ramban: Avraham was already promised the Land (12:7), and he did not ask for a sign at that point! Gur Aryeh adds another question
Rashi writes: "Alternatively, 'By what shall I know... (15:8)' - Avraham did not ask for a sign [regarding the Land], but rather inquired by what merit his descendants would endure there." Why does Rashi explain verse 15:8 at this point?
Gur Aryeh #1: We might wonder why in this verse, Hashem deems it Avraham's righteousness that he did not ask for a sign, yet Avraham does ask for a sign [regarding the Land] a mere two verses later! Rashi therefore clarifies that even that verse was an inquiry and not a request.
Gur Aryeh #2; Mizrachi: According to this approach, Avraham did not ask for a sign at all; nor was this what the Torah calls righteousness (for to ask would have been improper). Rather, Avraham was not perplexed by the promise of children (and that was his righteousness), whereas regarding the promise of the Land he remained perplexed as to by what merit it would endure. 1
Gur Aryeh adds that we thus understand why he earned reward for not questioning. (According to the first approach in Rashi, however, his reward should have been canceled out by his loss for asking about the next promise.)
Rashi writes: "Hashem answered, 'In the merit of the sacrifices." Is this really so? The Navi says, "It was not concerning sacrifices that I commanded your forefathers..." (Yirmeyahu 7:22).
Gur Aryeh: Avraham feared that just as the Kena'anim were exiled due to their sin, so too would his descendents be exiled at some point, "for there is no righteous man who never sins" (Koheles 7:20). Hashem responded that the Korbanos would atone for those sins, and they would be able to endure.
Rashi writes: "... in the merit of the sacrifices." Korbanos generally atone for sins committed b'Shogeg (unwittingly); what would atone for willful transgressions?
Gur Aryeh #1: Avraham was concerned only about sins committed b'Shogeg; if his descendants were to sin b'Meizid, they would deserve to go to exile.
Gur Aryeh #2: Once we know that Hashem atones for sins committed b'Shogeg, it shows that He is interested in our atonement. He will likewise grant us atonement for sins committed b'Mezid, through other means
Also see Maharal (Gevuros Hashem, Ch.8).