DEUT796 God promises to always take care of the downtrodden in society, and at the same time, commands Jews to help the downtrodden and give to them. (Exodus 22:21-22, Deuteronomy 24:19, this verse). Therefore, the act of giving Tzedaka fulfills both obligations at the same time -- God's and each Jew's. Since this is the only Mitzvah which accomplishes both goals in the same act, Tzedaka is unique, and God allows Jews to make giving conditional.