GEN827 If some give their tithes and some do not, a famine comes through drought; some go hungry and some have enough. Pirkei Avot V:10-11.  … A man could always find a way to circumvent his [tithe] obligation.  Only hashgahah, the watchful eye and care of Divine Providence, could know; only hashgahah could act. And so, if the crime became general, a general punishment would ensue to fit the crime; rainfall would decrease or come sporadically; there would be some drought; and the sinners would go hungry while others had food enough.  Thus the Israelite had to learn one lesson, willy-nilly: let him withhold his tithes from the Levite or needy, and eventually a drought would deprive him of all his crops. You save nothing by shutting your first on the share that belongs to charity. One way or another, Heaven will take its due.  The practice of giving a tenth of the harvest did not begin in Jewish history with the Torah’s commandment. Of themselves, the Patriarchs sensed a sacred obligation. When Abraham returned victorious from his battle with the four kings to save his captured nephew Lot, he have “a tenth of all” [this verse] the war booty to Melchizedek of Salem, who was “cohen to God most high.”  Genesis 14:18 as interpreted in Pirke d’R. Eli’ezer xxvi. SINAI3 117-8