Why did Lavan add the word "l'Ra'ah"?
Rashi: He meant that the prohibition of crossing this point was confined to doing so in order to do harm one another, but they were permitted to cross over to do business.
Riva citing R. Tam of Orleans, Moshav Zekenim: One of us is obligated to cross to help the other only if others do evil to the other.
It connotes that they will be punished if they do not cross to do evil!
Rashi, Riva: "Im" does not mean 'if,' rather, 'that [I will not cross].'
Riva citing R. Tam of Orleans, Moshav Zekenim: One will be punished if he does not cross to help the other when others do evil to the other. 1
About Lavan it mentions being Over (crossing) only the pile, and about Yaakov it mentions being Over the pile and the Matzevah!
Tosfos ha'Shalem (8, citing Chatzi Menasheh): Yaakov made the pile near himself, and Lavan made the Matzevah near himself. If Lavan crosses the pile, he already crossed the Matzevah, so he did not need to stipulate about it. If Yaakov crosses the pile, he did not yet cross the Matzevah, so he needed to stipulate about both. 1
Divrei Eliyahu and Kol Eliyahu: The pile is revealed. The Matzevah (Mitzpah) is concealed. Lavan can be Over (transgress, i.e. make war) only openly, but Yaakov can be Over openly (war) or covertly (Inuy, i.e. withholding intimacy from his wives).
Ha'amek Davar: The pile was a witness that neither cross it to make war. The testimony of the Mitzpah forbade Yaakov to take more wives. Lavan knew that Yaakov would not take from Benos Kena'an; he was concerned lest he [cross it, and] return to Aram to take another wife. 2
Malbim: Lavan had appointed two 'witnesses' over two matters (which Yaakov was not to transgress). Yaakov appointed but one witness. Refer to 31:49:2:3*.
They could have stipulated that neither cross the one closer to himself! If Lavan may cross the closer one, also Yaakov should be allowed to do so! (PF)
Yaakov could send a messenger (like Eliezer) to Aram to bring another wife! Perhaps this is as if he himself crosses. However, he could find a wife from elsewhere; Ramban (to 38:2) says that his sons married girls from Mitzrayim, Amon, Mo'av, Yishmael and Bnei Keturah! And Lavan also stipulated about afflicting his daughters! Why does he not explain that 'Over' is transgressing? Only Yaakov could potentially transgress the testimony of the Matzevah, for it obligated only him.
QUESTIONS ON RASHI
Rashi writes: "'Im Ani' - The word 'Im' here means 'that.'" Why not explain simply, that the mound will testify if I fulfill our covenant?
Gur Aryeh: How can a mound of stones testify (whether or not an event took place)? Rather, Lavan said, I am erecting the mound as a reminder that I committed not to cross over to harm you.