...According to the generosity with which you will give...Each man according to his ability to gift, consistent with the blessing of the L-rd your G-d."
Devarim 16:10-17
The Torah instructs us here about two different types of giving: one is described as stemming from a sense of "generosity," and the other is "according to your ability to gift."
These correspond to two different approaches toward charity giving, which often reflect the financial means of the individual. First, the Torah addresses a person who is not particularly wealthy, and who could easily justify using all his earnings for his own needs and the needs of his dependents. In this case the Torah appeals to his generosity, telling him to be benevolent and give even more than can be expected of him.
With the instruction to give "according to your ability to gift," however, the Torah demands more than generosity. Here the Torah addresses one who is affluent and is aware that G-d has blessed him with wealth well beyond his needs. Presumably, this person also knows that G-d provides sustenance for all humanity; some have the good fortune of earning it on their own, and some must rely on the generosity of others in order to receive the sustenance intended for them. He therefore understands that G-d has not only provided him wealth to support himself, He also appointed him custodian over funds intended for others, and it is therefore logical for him to be charitable. In this instance, it is not necessary for the Torah to expect that he be generous--for he already knows that the money is intended for charity, and does not even regard it as his own. Instead, the Torah addresses the cordiality with which he gives, and instructs him not to give grudgingly, but "according to his ability to gift"--with the warmth and friendliness of a person giving a gift to a friend.
--From the forthcoming Light/Points