The Yahrtzeit Of A Great Man

Today is the yahrtzeit of the tremendous gaon Rav Ben Tzion ben Eliyahu Hakohen Kahn ztz"l [a survivor of the camps who made a new life for himself and built a family in Williamsburg] the author of the Avnei Tzion, one of the most amazing sfarim I have ever seen.

Li-ilui nishmaso I will bring three kushyos of his from his sefer Korban Tzion:

1] Rashi [Succah 27b] says that if a person has a portion of less than a prutah it is not considered לכם.

This is difficult because we find regarding machatzis ha-shekel that people give in order that everybody should have a portion in the korbanos tzibbur [עי' חינוך מצוה ק"ה וב"מ נ"ח תורמין על האבוד או על הגבוי וכך פסק הרמב"ם שקלים ב א] that each person is not going to have a portion in every korban that is worth a prutah [because there are so many people everybody's portion is less than a perutah]. So how do we get a portion in the korbanos tzibbur?

2] In a similar vein: The Mishna in Shkalim [2, 2] says that if one takes a shekel from the funds of the Beis Hamikdash and pays his half shekel dues and the money is used to buy korbanos - he is עובר באיסור מעילה. But wait! He is getting less than a shaveh prutah's worth of benefit from the korban brought [because there are so many people his portion is less than a perutah]?

3] The Gemara [Shabbos 157] says that one may do הפרת נדרים on Shabbos. But how is this different from הפרשת תרומה which is אסור משום מתקן - here also he is מתקן himself when he does הפרה?