The Torah tells in Parashat Vayeitzei of Yaakov’s vow which he made as he left Canaan to flee from his brother, promising to tithe all the material assets with which God blesses him, upon his return to his homeland – “ve-khol asher titen li aser a’aserenu lakh” (28:22). The Gemara, in Masekhet Ketubot, understands the poetic form “aser a’aserenu” as indicating that Yaakov refers here not to one-tenth of his assets, but rather to one-fifth (two-tenths). On this basis, the Gemara views this verse as a Biblical allusion to the law enacted by the Sages in Usha, “Ha-mebazbez al yebazbez yoter mi-chomesh” – one should not donate more than one-fifth of his assets to charity. Although this provision was enacted by Chazal, Yaakov’s vow is seen as an ancient source for this law.
A number of Acharonim raised the question of how the Sages could enact such a law, which seems to override an explicit prohibition in Sefer Devarim (15:7) against withholding charity from those in need: “Do not harden your heart or shut your hands from your brother, the pauper.” It is generally assumed that one must spend any amount of money necessary to avoid committing a Torah violation (as opposed to fulfilling mitzvot, for which one is not required to spend a significant percentage of his assets), and this is indeed the explicit ruling of the Rama (O.C. 656:, Y.D. 157:1). Seemingly, then, it should be obligatory to spend any amount of money needed to support (at least at the standard of minimum subsistence) a pauper in dire need of charity, to avoid violating this Torah prohibition.
Rav Asher Weiss cites various different answers to this question. First, he notes that several halakhic authorities – including Rav Yaakov Emden, in She’eilat Ya’abetz (1:3) – imposed a surprising limitation on the rule of “ha-mebazbez al yebazbez yoter mi-chomesh.” In their view, this means that one should not allocate more than 20 percent of his assets to charity. If, however, we are approached by a destitute pauper in desperate need of help, then we must pay any amount of money needed to assist him, even if this exceeds 20 percent of our possessions. According to this opinion, the 20-percent limit clearly does not conflict with the Torah prohibition against withholding money from the poor, as the Torah prohibition applies only when we have a needy individual in front of us who needs help. Rav Weiss notes, however, that the Rambam, in Hilkhot Matenot Aniyim (7:1), strongly indicates that the rule of “al yebazbez yoter mei-chomesh” applies under all circumstances.
Another approach that can be taken is based upon the position taken by several halakhic authorities, including the Chatam Sofer (notes to Shulchan Arukh, O.C. 656) and the Peri Megadim (O.C. 656), that one must pay all his money to avoid only actively violating a Torah prohibition. If a Torah prohibition forbids an inaction, however, then one is not required to sacrifice a large portion of his assets to comply with the prohibition and perform the required action. Therefore, although one must pay any amount of money necessary to avoid committing a forbidden act, one is not obligated to pay all his money to support a needy pauper. Rav Weiss notes, however, that this position is not universally accepted, as the Rivash (387) maintains that one must pay all his money to avoid any prohibition, even a prohibition which forbids refraining from a given act.
A fairly simple answer is suggested by the Maharil Diskin (1:24), who claims that the prohibition against withholding charity is linked to, and dependent upon, the affirmative command to give charity. Hence, the prohibition applies only to the extent to which the affirmative command applies, and thus if Chazal limited the affirmative command to 20 percent of one assets, then one does not violate the prohibition by not exceeding this amount.
Rav Weiss also offers his own answer, claiming, quite simply, that one who does not exceed the amount of 20 percent is not considered as “hardening his heart” or “shutting his hands.” Since he gives the amount which Chazal established as the maximum sum of charity one should give, he does not show any apathy or indifference towards the poor person by not giving more, and therefore he does not violate the prohibition against withholding charity.