Some Poskim maintain that if an opportunity to do a [non obligatory] Mitzvah arises that requires the spending of money (e.g. marrying off an orphan) and one would not be able to afford the Mitzvah otherwise, he/she may use Ma'aser money to perform the Mitzvah.
However, if one is obligated to perform this Mitvah (e.g. buying an Esrog and Lulav) then Ma'aser money may not be used according to all Poskim. (See Shu"t Chasam Sofer Yoreh Deah Siman 231 for a more lengthy explanation of the difference between Mitzvos that are required and those that aren't)
2) The best thing to do when separating money for Ma'aser is to stipulate that it may be spent how you see fit, thus it will be permitted to use it for any Mitzvos that arise. (Shu"t Bais Dino Shel Shlomo Yoreh Deah Siman 1; See also Shu"t Chasam Sofer ibid. and Birchei Yosef Siman 249:4)
If one's custom is to always give his/her Ma'aser money to poor people, according to many Poskim, the above stipulation will not work and he/she must only use the Ma'aser money for poor people. (See Shu"t Tzitz Eliezer Vol. 9 Siman 1. See also Shu"t Levushei Mordechai Yoreh Deah Siman 179).
Important Numbers
