כתובות נ.
"המבזבז אל יבזבז יותר מחומש"
The Gemara brings a beraisa that explains the reason we prevent someone from giving away more than a fifth of his assets to Tzedaka [רש"י], is to ensure that he himself doesn’t ultimately become poor & יצטרך לבריות. The [או"ח תרנו] רמ"א paskens that even for a מצוה עוברת one may not be מבזבז more than a fifth of his worth, but only for a מצות עשה; for a לא תעשה one must give up all his money not to be oiver. In ביאור הלכה [שם], the ח"ח brings the Gemara [קידושין כט] that discusses a case where one owns only 5 סלעים & needs to do פדיון הבן on his son & on himself. Who is first? The ח"ח says this seems to be contrary to what we just learned, if he only has 5 סלעים to his name he shouldn’t be מחוייב to spend it all on the mitzvah? עיי"ש. The ח"ח [אהבת חסד ח"ב, כ'] brings from the שיטה מקובצת that this תקנה does not apply to distributing one’s money for החזקת התורה & one may even donate more than a fifth of his worth. Since the giver shares in the לימוד התורה of the receiver, it is mutar. Furthermore, if one is anyway spending money on frivolous & unnecessary items, this rule does not apply to him either. עיי"ש באריכות. The [שו"ת ח"א סי' כד] מהרי"ל דיסקין asks why is one patur to spend more than a fifth on צדקה like החזקת התורה because if one withholds tzedakah he is oiver the Lav of לא תאמץ (עיין תוס' לעיל מט: ד"ה אכפייה) & we know that for a Lav this Takana does not apply? He answers that in this case the Lav is only a שמירה for the עשה, so if there is no עשה, there is no לאו.
In Sefer [עניני שבת עמ' נ"ט] מרפסין איגרא they asked ר' חיים קניבסקי זצ"ל the following: the מגן אברהם says [או"ח רמ"ח, ס"ק טז'] if one is traveling in the מדבר with a caravan & is overtaken by bandits, if he is concerned that all his money will be stolen & he will be left with nothing, this is considered פיקוח נפש & it is דוחה שבת. If so, how can the רמ"א say one must give up all his money not to be oiver a לאו, by doing so he will be bankrupt & in סכנת נפשות? ר' חיים answered that while the תקנה does not apply by a ל"ת & one must be מבזבז more, he must still leave over money for himself to live on, in order not to be in the מצב of ס"נ. It is brought [חידושי הגר"ח ס' קכט'] that someone once came and asked ר' חיים (סלאוויציק), if פריעת בעל חוב is a מצוה, one should not have to be מבזבז more than a fifth to pay back a loan? ר' חיים answered that when he is being מבזבז his money to pay back the loan, he is not spending his money, but rather he is just returning the lender’s money!
It says in Mishlei [ד'-ז'] "ראשית חכמה, קנה חכמה". The אבן עזרא explains, if need be, spend all your money to acquire חכמה (תורה). Maybe this is why when הלל הזקן earned 1 טרפעיק [יומא לה:] he sent half of it home for living expenses & the other (much more than a fifth) he used for entry into the בית המדרש. The גליון הש"ס here on our amud, brings the ירושלמי that this is really a הלכה למשה מסיני, but it was forgotten over time until it was reestablished again in אושא.