To whom must I give Tzedoko?
Any Meshulach that comes one’s way and asks for Tzedoko, it is a Mitzvah to give them Tzedoko. One is not allowed to turn them away without giving them even the smallest amount. This is based on a Posuk in Tehilim, Al Yoshoiv Dach Nichlam, one can’t turn them away empty-handed so that they are not insulted or hurt. The Rambam writes that one can fulfill this Mitzvah even by giving them a Kigroiges, which is the size of a fig, or even an actual fig. This means that it could be a small amount of money, and does not even have to be a shove Pruta. This is not about the Mitzvah of Tzedaka; this is about not offending the person. Of course, sometimes one is bombarded with people all day long, while learning and davening, or at home. Therefore, if one is in the middle of a Mitzvah, even if one is busy with their children which is also a Mitzvah, they do not have to do another Mitzvah. However, if one is in the middle of learning, they have to interrupt their learning and give the person Tzedoko.
https://halacha2go.com?number=2