And Lavan said to Yakov, "Just because you are my relative, you should work for me for free? Specify what your wages will be!" Lavan had two daughters; the older was named Leah and the younger was named Rachel... and Yakov loved Rachel, so he said, "I will work for you seven years for Rachel, your younger daughter." (Bereishit 29:15,16,18)
"For Rachel, your younger daughter" -- Why was it necessary for Yakov to mention all of these identifying details? Yakov knew that Lavan was a cheat. He therefore said to him, "I will work for you for Rachel. In case you say `I will bring another girl named Rachel from the street,' I add `your daughter.' And in case you say, `I will take Leah and change her name to Rachel, I add `younger.' "But, in spite of all this, Lavan tricked him. (Rashi 29:18, quoting from Bereishit Rabba)
Yakov was well aware of Lavan's dishonest character. When he met Rachel for the first time, he told her that he was "the brother of her father." Rashi, noting that he was not Lavan's brother but his nephew, quotes the Midrashic interpretation: Yakov meant to say, "If Lavan tries to deal treacherously with me, I am capable of being his brother in deceit!" Although he was the ultimate Ish Tam [innocent, guileless man], Yakov apparently had the ability to meet a scoundrel head on and beat him at his own game. That is why he was so cautious when he was about to enter into his first business deal with his unscrupulous uncle.
Yakov knew that Lavan was "only" a Ramai -- a deceitful person. He was not a Shakran -- an outright liar -- but a deceiver. He would not simply come to Yakov at the end of his seven years of hard labor and say, "The deal is off; I've changed my mind. Here is Leah, take her or leave her." Rather, he would concoct some sort of explanation to justify his actions. Because of this, Yakov was confident that he would be able to deal with Lavan. Yakov thought that he had covered all the possibilities when he specified that he expected "Rachel, your younger daughter." Nevertheless, we find that Lavan managed to outsmart him.
If Lavan was not an overt liar, how is it that he did indeed replace Rachel with Leah? Is there any way it can be said that Lavan found a loophole in Yakov's carefully worded request? How did he succeed in fooling Yakov without breaking the literal meaning of their agreement?
"And Zilpah gave birth" (Bereishit 30:10) - With all the other wives of Yakov the Pasuk says "she conceived and gave birth." Only concerning Zilpah is conception not mentioned. This is because she was younger than all of the other wives. Since she was still a young girl, her pregnancy was not so recognizable.
Lavan purposely gave Leah such a young maid in order that Yakov should not realize that his bride was really Leah, for the common practice to give a young maid to the younger daughter and an older maid to the elder daughter. (Rashi, Bereishit 30:10)
Bilhah and Zilpah were not merely maidservants who were bought by Lavan to serve his daughters. Rashi points out later in the Parsha (31:50), that Bilhah and Zilpah were actually *themselves* daughters of Lavan from a concubine.
We can now understand how Lavan managed to do what he did with mere "trickery", as opposed to unadulterated lying. When Leah was given to Yakov, she was accompanied by Zilpah (29:24). Zilpah was indeed Lavan's youngest daughter, as Rashi explained. Lavan was to be suspected of renaming another daughter "Rachel" since that was only a deceitful trick and not an outright lie. Lavan nicknamed Zilpah, his youngest daughter, "Rachel" (which, after all, means "little lamb" in Hebrew). Lavan had thus, in the literal sense, fulfilled the conditions of "Rachel," "daughter" and "youngest daughter" by giving Zilpah over to Yakov!
Lavan then could justify his warped interpretation of the provisions of the agreement by explaining to Yakov that it was unheard of in their place to have a younger sister wed before her older sister (29:26). It was therefore only natural to assume, said the wily Lavan, that Yakov meant to request the younger of Lavan's two concubine daughters as a maidservant, not the younger of his two full-fledged daughters as a wife. The agreement was thus carried out in full, claimed Lavan, by giving Yakov Zilpah as a maidservant for Leah!