R’ Moshe says mefurash in a tshuva that ma’aser ksafim nowadays is only a “minhag tov” & not a chiyuv.
All agree that one whos expenses exceed his income is pattur from giving maaser.
Sjs,
If you are looking for a “chumra”, the Vilna Gaon holds (brought by the Chofetz Chaim in Ahavas Chesed) that one must give 20% – m’ikar hadin! Not 10%. 🙂
As someone very correctly pointed out, all these halachos are brought in Shulchan Aruch.
R’ Chaim Kanievsky also has a full peirush modeled after the Mishna Berura called “derech emuna” which covers Hilchos Tzedaka & Hilchos Maaser – two different things.