According to Kabbalah, when a person gives charity, he causes G-d's countenance to shine. The Sefirah of Malchut gets its drawdown of Divine Light from the other Sefirot, it has nothing of itself. This is similar to a poor person, who has nothing without receiving from others. There is a Kabbalistic concept of creating an arousal above by our acts below. In this case, charity is the arousal that brings Divine Light into the world. Kabbalh talks about unities. In this case there is peace between extremes, in action it is charity, and in Divine Light it is the higher worlds being brought down into lower limited worlds. Thus G-d's light shines, so to speak, when it is aroused by charity in this world.

Based on Igeret Hakodesh Chapter 30 Rabbi Shneur Zalman of Liadi