How could Yackov Lie it is גניבת דעת to say
אָנֹכִי עֵשָׂו בְּכֹרֶךָ
How was he allowed to make this statement?
Taken at face value, this statement is an outright lie. Though it is clearly intended to make Yitzchak think that Yaakov was actually Eisav, it is still unsettling that Yaakov (who is often thought to be the paradigm of truth) could lie like that.
Therefore, Rashi gives his interpretation of how to read Yaakov's words:
אנכי המביא לך ועשו הוא בכורך
I am [the one who is bringing your food and] Eisav [is] your firstborn.
This does seem a little stretched, though, and thus I give you the answer of Abarbanel and Alshich who interpret his words: "I am your firtsborn, Eisav", that is, I represent the position of Eisav, your firstborn, ever since he sold to me the bechora; since then I am him regarding all things relating to the bechora (including the b'rachos).
I have no source, but I have always learned as follows:
Contrary to popular belief, the main issue with lying has less to do with the words you use and more to do with the message you are conveying. If a friend at your home asks you for $5 and you say "I don't have any money on me" when the money is in the other room, that is sheker even though your words are true because the message that you are trying to convey is that you have no money to lend, which is false. On the other hand, if you have $5 in your pocket and say "I don't have any money on me" because you need that $5, you have not conveyed sheker.
However, a higher level of midvar sheker tirchak would oblige you to use words which are true as well, even though you are already conveying emes. (I once had a Rebbi who was practicing emes. When someone asked him if he had a pen, he would answer "I have a pen, but it's at home".)
It was Yaakov's right to obtain the brachos. It was not sheker to convey the idea that the person talking to Yitzchak should receive the blessing. However, Yaakov wanted to use use words that are true within themselves.
דף על הדף קידושין דף יח עמוד א
מכח דין זה שלעשו היה דין ישראל מומר, נתבארו כמה דברים: בתורתך שעשועי (תולדות) ביאר לפי זה, כיצד הותר ליעקב אבינו לגנוב דעת אביו ולומר (בראשית כז, יט) 'אנכי עשו בכורך', אלא משום שהיה לעשו דין ישראל מומר שאסור לאכול משחיטתו, ונמצא שבמעשה זה הציל יעקב את אביו מלאכול משחיטתו האסורה של עשו, ולאפרושי מאיסורא מותר לשקר, דלא גרע ממה שמותר לשקר מפני דרכי שלום.