Rashi (Bereishis 30:10) says that Zilpah was younger than Bilhah. Why did Lavan give the younger maidservant to Leah, his older daughter (and vice-versa)?
Rashi (30:10): He did that to support his trickery, so that Yakov should believe that he was marrying Rachel (and not Leah). 1
Rashi: Since it was customary to give the older maidservant to the older daughter.
Since it says that he gave Zilpah "l'Leah", there was no reason to write "Lah"!
Hadar Zekenim: He made Yakov think that he was giving her to Rachel. 1
However, it says "Lah" superfluously also when he gave Bilhah to Rachel (verse 29)! This is difficult.
Why does it say that he gave Zilpah "Shifchah"? It says (verse 29) that he gave Bilhah "l'Shifchah"!
Tosfos ha'Shalem (4): Zilpah was very young, and not so proper to serve.
Ohr ha'Chayim: A Midrash teaches that she already was Leah's Shifchah. Rachel and Leah inherited Bilhah and Zilpah from their mother's Kesuvos. 1 "Bito" is extra, to hint to how she had acquired her. "Lavan" is extra, for only he made it seem that it is a gift.
If so, why does it say l'Shifchah regarding Bilhah? (PF)