The Gemara in Baba Kama 56b says that one who is osek b’mitzvah is exempt from giving tzedakah, even if a poor man approaches him. Many Achronim question why the rule of osek b’mitzvah patur min ha’mitzvah can only exempt one from another mitzvah, but it cannot permit one to transgress a lo sa’aseh. When a poor man asks someone for tzedakah, there is a mitzvah to give him tzedakah (as it says: pasoach tiftach) and a lo sa’aseh not to give him (as the pasuk says: lo s’ametz). So how can the Gemara say that one who is performing a mitzvah is exempt from giving tzedakah?
The Ohr Somayach, the Imrei Binah, Reb Elchonon Wasserman, and the Steipler answer that the lo sa’aseh of lo s’ametz is connected to the mitzvah of giving tzedakah; and whoever is exempt from the mitzvah of giving tzedakah is not prohibited by the lo sa’aseh either. Therefore, when one is involved in performing a mitzvah he is even exempt from giving tzedakah.
According to the Nesivos (as explained earlier) that the exemption differs depending on whether one is performing a voluntary or obligatory mitzvah, the answer of the Achronim regarding the mitzvah of tzedakah will only apply when one is performing an obligatory mitzvah. This is because when one is involved in the performance of an obligatory mitzvah, all other mitzvos are considered as if they are not mitzvos for him. Thus even the mitzvah of tzedakah does not apply to him. Therefore, the lo sa’aseh does not apply to him at that time as well.
On the other hand, when one is involved in the performance of a voluntary mitzvah the other mitzvos remain mitzvos even for him, except that he is allowed to continue performing the mitzvah that he first started. Therefore, regarding the mitzvah of tzedakah that is accompanied by a lo sa’aseh, one who is involved in performing a voluntary mitzvah will be obligated to stop and perform the mitzvah of tzedakah, so as not to transgress the lo sa’aseh associated with not giving tzedakah.
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