How could Yaakov (who was the epitome of Emes), lie so blatantly to his father? stay
Rashi: He meant "I am (the one bringing to you), and Esav is your firstborn." 1 It was a white lie (since it was the truth; only that Yitzchak understood it differently). 2 Likewise, when he added, "Asisi ka'Asher Dibarta Elai," he had indeed done many things like`Yitzchak said - in which case that too was a white lie.
Chizkuni #1: For a temporary need, one may deviate from the truth, like Avraham said, "Say that I am your brother" (20:13).
Chizkuni #2: Yaakov is the Bechor, for he bought the Bechorah from Esav. Ohr ha'Chayim, Malbim - since he is the Bechor, he is Esav! 3
R. Bechayei: You give the Berachos due to the Bechorah. Since I bought the Bechorah from Esav; I am in place of him.
Asarah Ma'amaros Perek 18, #1: Yaakov was created from the first drop of Zera. 4
Asarah Ma'amaros Perek 18, #2: Yaakov was the Bechor to Shamayim (his Neshamah was created first). Esav was the physical Bechor.
Presumably, Rashi learns this from the fact that Yaakov said "Anochi" and not 'Ani.' Note that (in 27:32), in answer to the same question, Esav replied "Ani Bincha Bechorcha Esav." (EC) Also see Gur Aryeh; refer to 27:19:1.2:1. Oznayim la'Torah - the Torah writes "mi'Devar Sheker Tirchak" (Shemos 23:7) to teach us that, if one must lie (because of Darkei Shalom or in a case such as the current one), one should distance the lie as much as possible by at least expressing oneself in a way that has dual implications.
In fact, Yaakov would have been permitted to lie directly (bearing in mind that he was fulfilling his mother's express orders and that, as the commentaries explain, he was performing a Mitzvah, by taking away from Esav that which he was not worthy of). It is only because he was the Ish Emes that he minimized the deviation from the truth in this way.
Ohr ha'Chayim: You commanded Esav, because [you thought that] he is the Bechor. Since I am the Bechor, it is as if you commanded me. I did like you commanded!
Why did Yaakov say "Anochi Esav Bechorecha," whereas Esav said "Ani Bincha Vechorecha Esav" (27:32)?
Malbim: One says first what [Chidush] is primary to him. Yaakov was the Bechor, due to the sale. His Chidush was, I am Esav; it is true, since he is the Bechor. For Esav, 'I am Esav' is no Chidush. His (false) Chidush was to claim to be the Bechor, even though he sold it. "Anochi" stresses the speaker's identity. Yaakov said so, to stress that he is Esav. Esav had no need to say so; his Chidush was the adjective (I am the Bechor).
Why did Yaakov mention the Bechorah?
Ha'amek Davar: This Berachah is only for this world. It is more relevant to Bechorah than matters of Kedushah are. All the matters of Kedushah that used to belong to Bechoros were taken from them and given to others. See Ha'amek Davar to 48:14. 1
Perhaps he refers to Reuven losing kingship, Kehunah and being two Shevatim). However, in 48:14 he says that Efrayim came first for supernatural matters, but Menasheh still was first in natural matters.
QUESTIONS ON RASHI
Rashi writes: "'Anochi Esav Bechorecha' - It is I who brings to you; and Esav is your firstborn!" Why interpret Yaakov's words in this way?
Gur Aryeh: HaSh-m hates falsehood; how could Yaakov attain His blessings by means of falsehood? By interpreting in this way, Rashi explains that Yaakov did not lie.
Rashi writes: "'Anochi Esav Bechorecha' - It is I who brings to you; and Esav is your firstborn!" How is this interpretation alluded to in Yaakov's words?
Gur Aryeh: Yaakov used the word "Anochi," whereas Esav later used the word "Ani" (27:32). Grammatically, when the word 'Ani' appears in construct, the letter Alef has a Chataf-Patach, but when it appears at a pause, it has a Kamatz. 1 If Yaakov had used the word 'Ani,' he would have had to use a Kamatz at his pause, and Yitzchak would understand that it was not Esav! Yaakov chose the word "Anochi," which is vowelized the same way in either case.
For example, see 27: 24,34.