Tzedaka: Testing God

No-one ever became poor by giving charity. [Gittin 7a; Yoreh Deah 249]

The basic law of charity is to give one-tenth (ma'aser) of the income.  A higher level is chomesh or one-fifth [Yoreh Deah 249:1]. 

The Talmud teaches that one should give tithes in order that one become wealthy [Taanith 9a].

There was a wealthy man who commanded his son to give tithes. The field yielded one thousand kurim a year.  After his passing, the son felt that one hundred kurim was too much to give away so he did not tithe that year.

The following year, the yield was one hundred kurim. When confronted with the poor yield, his family said "Last year you gave tithes, the field was yours and the tithes G-d's.  This year the field is G-d's and the tithe yours." [Tosafot]

The Talmud further states that one is permitted to test G-d in giving tithes, to see whether one's income will increase. [Tosafot]  Conversely, any loss of income a person will sustain is decreed on Rosh Hashanah.  If he merits, it will go to charity [Bava Basra 10a]. "The door that does not open for charity will open for the doctor" [Yerushalmi]

Source: "The Master of Prayer"  by Rebbe Nachman of Breslov, commentary by Rabbi Aryeh Kaplan