When a person gives charity with the intent of doing God's will, he becomes the instrument through which the trait of divine kindness itself gives charity.
At that moment, he is God's "right hand" working through human agency, and the Shekhinah, God's presence, dwells within him.
The halakhah allows one to give at most a fifth of one's income to charity (see Shulchan Arukh, Yoreh Deah 249).
That fifth contains all of the strength and life that a person invested in earning it, and so it encompasses the other four parts.
Because halakhah prohibits a person from donating more than a fifth of his income to charity, that amount represents all of his income.
Moreover, since this money represents all of his hard work, it epitomizes his entire life and being.
As a result, these rise in holiness.
The mitzvah of charity has the same effect as the sacrifices did: it raises the world and brings it closer to God.
–Rabbi Adin Steinsaltz
From Understanding the Tanya, p. 176 by Rabbi Adin Steinsaltz