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The Posuk in this weeks Parsha states that if one does not have the financial means to afford the two turtle-doves or the two young pigeons required for a certain Korban, he should instead bring as his offering for his transgression one tenth of an Eiphah of fine flour. The Chafetz Chaim once commented that we can see from the Posuk how the Torah set different standards for a rich person as opposed to a poor person. We see that a wealthier person must bring an offering that is worth more; therefore, the rule is that if a wealthy person brings a poor person's offering, it is not valid and he still has an obligation to bring another Korban for his sin which corresponds to his stature.
This rule also applies when one gives to charity. The more money one has, the more one has an obligation to give. Based on tradition, the Halacha recommends that everyone give at least one tenth of his income to charity. Obviously, then, one who has a much higher income must give much more money to charity. This idea also applies to other aspects of Judaism. The greater one's intellect, for instance, the greater is one's obligation to learn and share his wisdom with others.
Rabbi Shlomo Zalman Lipschitz used to give money to poor people even though as a result, he would often be left without any money for himself. When people asked him why he didn't leave any money for himself, he would reply that people trust him and he can therefore always buy what he needs on credit. But no one will want to sell to this poor person on credit, and therefore he is in greater need of this cash. We can learn from this story of Rabbi Lipschitz that if someone with barely any money was so willing to give to people even less fortunate than himself, surely we, who are generally well provided for, can afford to give a significant portion of our income to charity.