Placing 'Esav' (really, Yaakov) master over his brother was the seventh Berachah (See above, 27: 28-29). Why did Yitzchak mention it here first?
Rashi: He was telling Esav that whatever Berachah he would confer upon him was virtually worthless, since, based on the principle that 'Whatever a slave owns belongs to his master,' whatever he would give him would now belong to Yaakov.
Who were the 'brothers' whom he gave to Yaakov as slaves?
Seforno, Targum Yonasan 1 and Ramban (citing the Ibn Ezra): The Bnei Yishmael and the Bnei Keturah.
Ramban: It refers to all the sons of Esav, who in addition to being subservient to Yaakov, would also be his slaves.
What does the verse mean, "and grain and wine, I supported him"?
Sha'arei Teshuvah (of R. Yonah, 1:42): It is as if there is a Beis - with grain and wine, I supported him.
Why does it say "va'Ya'an... va'Yomer"?
Ohr ha'Chayim: This is only what he told Esav, but it was not true. Refer to 27:37:153:1.
Ha'amek Davar: "Va'Ya'an" teaches that it was in a crying, merciful voice.
What was Yitzchak's answer why he cannot bless Esav as well?
Ohr ha'Chayim: Yitzchak was making excuses not to bless him, for he understood that Esav is Yaakov's enemy, and he should be cursed - "Orerecha Arur" (27:29).
Malbim: Your Berachah was to supply Yaakov's income and protect him, so he can engage in Torah. Now that I gave power, authority and wealth to Yaakov himself, he does not need you at all. It is no Berachah to give them to you!
Ha'amek Davar: "Lechah" with a Hei at the end implies 'you, and no other' (refer to 27:7:152:1). Had I given the Berachos to you, I could have blessed Yaakov with spirituality. However, now that I blessed him, there is nothing to bless for you!
QUESTIONS ON RASHI
Rashi writes: "'Behold, [I have made him] master' - ... and whatever a slave acquires belongs to his master." If so, what did Esav intend to gain with his response, "Do you have one blessing for me" (27:38)?
Gur Aryeh: Esav argued that Yitzchak could still grant blessings in the reverse of Yaakov's blessings, which Yaakov would have no part in. And in fact, Yitzchak blessed Esav with fertile lands (27:39), 1 while Yaakov had no desire for unclean lands outside of Eretz Yisrael. Esav was also blessed to live by the sword (27:40); the antithesis of Yaakov's traits of peace and truth.
Esav's blessing would stem from the nature of his land, not as a gift from HaSh-m. See Gur Aryeh to 27:39. (EK)