Q. I’m planing to invest the capital obtained (from a property that I inherited from my parents – see last question) and live of the interests. The amount to be received monthly is small and barely covers my needs. Do I still have to give maser?

A. Poskim disagree if someone in monetary constraints is exempt from donating ma’aser. Some rule strictly and require ma’aser even of the poor unless they can’t even purchase bread and water. The lenient maintain that if one has to rely upon others to satisfy basic needs, and has no extras or luxuries, then he need not give maaser. See Teshuvos V’Hanhagos 1:560, Kuntres Kol Torah Choberes 39 quoting Horav Shlomo Zalmen Auerbauch Zt”l.
However, the Talmud (Gitin 7a,b) maintains that the poor should also give tzedaka (which is a separate mitzva from m’aaser,) as Rav Yosef says; the destitute will be rewarded for giving charity and will cease being poor.
When someone accepted giving ma’aser as a neder, even if he has a difficult time with parnosso, he still is obligated to keep his promise (Tzedaka Umishpat 5: 20, see also Chasam Sofer C.M. 120 that Ya’Akov Avinu took on as a neder to give ma’aser even in need)
Minchas Yitzchok (6: 101) quoting Tashbatz recommends separating the tithe, and using some of it for his family needs.
Horav Shlomo Miller’s Shlit’a opinion is that you should give as much as you can afford.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a