What exactly did Ya'akov undertake to do with the stone?
Rashi: Refer to 28:18:3:1*.
Targum Yonasan: That it will be the cornerstone of the Beis-ha'Mikdash, where all of Yisrael will serve Hashem.
What did Ya'akov mean when he said that he would take a tenth of all that Hashem gave him?
Rashbam: He undertook to take Ma'aser from animals that he would amass, to offer them up as Olos.
Moshav Zekenim, Hadar Zekenim, Da'as Zekeinim: He enacted to take Ma'aser 1 from money. He tithed even his children. Since Efrayim and Menasheh are considered like Reuven and Shimon, it is as if he had 14 sons. Four 2 were Bechoros, which are exempt from Ma'aser. He gave Levi 3 to be Ma'aser (Bereishis Rabah 70:9),
Tosfos ha'Shalem (18, citing R. Efrayim): What is so praiseworthy about returning to Hashem a 10th of what Hashem gives to him? It is even if he will get only through great exertion.
Hadar Zekenim: Perhaps Menasheh was not a Bechor. Moshav Zekenim rejects this. The Torah (Bereishis 41:51) calls him a Bechor! (Perhaps he was Yosef's Bechor, but Asnas had a child beforehand - PF.) Rather, he was a Bechor; Reuven's Bechorah was given to Yosef (Divrei ha'Yamim 1:5:1), so there were only four. (We could say simply that one cannot split a person, therefore Yakov gave one son, but it is better to show how a Tzadik's words recorded in the Torah are fulfilled exactly (refer to 48:6:1:1****). Perhaps a 10th of Yakov's daughters married Levi, so also they were considered Ma'aser - PF)
Hadar Zekenim explains why he chose Levi. The Perush of R. Chaim Paltiel utterly astounds me. Also his question I do not understand. Even Ma'aser Behemah, which must be the 10th to leave the pen, need not be the 10th born!
Why did he use the double expression "Aser A'asrenu"?
Ba'al ha'Turim: To teach us that that one may not give more than a fifth (two tenths) of one's money for Tzedakah. 1
Tosfos ha'Shalem (3): He enacted Ma'aser Rishon and Ma'aser Sheni.
Tosfos ha'Shalem (15, citing R. Efrayim): This hints to "tithe, in order that Tis'aser (you will become rich - Shabbos 119a)."
Moshav Zekenim: He will tithe even the food he eats every day.
See Ba'al ha'Turim.
Kedushah does not take effect on something not yet in the world!
Tosfos ha'Shalem (18, citing R. Efrayim): However, a Neder takes effect on it.
"Ha'Even" is feminine. Why does it say "Yihyeh" (masculine)?
Bechor Shor: Its place will be Beis Elokim. ('Makom' is masculine.)
Tosfos ha'Shalem (4, citing R. Efrayim): "Even" can be feminine or masculine.
רש"י: כתרגומו אהי פלח עלה קדם ה' וכן עשה בשובו מפדן ארם: מה בא רש"י לפרש?
גור אריה: "בית אלוקים" כאן לא מתפרש כפשוטו [-בית של אלוקים], אלא בית לעבוד בו את ה'.