Giving tzedakah, charity, is arguably the most important positive mitzvah. How much should one give? The classical sources say that 10 per cent is average; giving 20 per cent is the best way to do the mitzvah. (Rambam: Laws of Gifts to the Poor, 7:5). Modern writers translate this into a one-off gift of a fifth of one's assets and then a fifth of one's annual income. May one give more than a fifth? There is no clear agreement in the sources.
A decree was made at Usha, a town in the Galilee, that one should not lavish more than a fifth of what one has on charity. The reason given in the Talmud is that one might thereby bring oneself to poverty and dependency on others. Later halachic sources make exceptions to this principle if the cause is life-saving, for example ransoming of captives, if the donor is on his death bed and, according to some, if he is wealthy and in no danger of becoming poor through giving a larger percentage of his assets and income. It is hard to believe that the halachah would not support the Gates-Buffet initiative to have the world's billionaires give away half of their wealth.