How could Lavan claim "Lo Ye'aseh Kein bi'Mekomeinu ..."? Did he not know that when he made the agreement with Yakov?
Ramban, Ohr ha'Chayim #1: He claimed that, although he personally, was willing to bypass the local custom, the people will (Seforno
This is why Rashi says that "Nitnah" is plural (Tosfos ha'Shalem (27:1, citing R. Efrayim).
If one was Mekadesh "your daughter" and did not specify which, we do not say that surely, the father gave the older (Kidushin 52a)!
It seems that "we do not give the younger before the older" is not a universal custom. 1 (See Tosfos 52a DH v'Hilchesa.)
However, the Rashbam (Bava Basra 120a DH l'Halan) explains that presumably, Benos Tzelafchad married in order of their age, and the Bach and Shach (YD 244) bring this l'Halachah.
Why was Yakov not concerned for the custom?
Ohr ha'Chayim: He held that a father is allowed to override it. When I asked for Ishti, you knew that I meant Rachel; you should have said that the custom impedes, and I may choose to take Leah, or to wait for her to marry someone else!