We are supposed to give between 10 and 20% of your income to צדקה.
Does this mean that you are not allowed to give over 20% if you have enough money to do so? If so, why not? (this is more of a theoretical question for me)
Kesuvos 50a says that you should not give more than 20% in order not to become dependent on others. However Shulchan Aruch Yoreh Deah 249:1 says that if you can afford it you may give more. אם ידו משגת יתן כפי צורך העניים. The Igros Moshe Yoreh Deah 1, Siman 143 says that in actuality unless it is for Pikuach Nefesh not to give more than 20%.
R. Shneur Zalman of Liadi writes (in Iggeres Hateshuvah 3 and Iggeres Hakodesh 10) that the 20% limit applies only to a person who hasn't sinned, or who has done so but then fasted the prescribed number of fasts as atonement. For someone who is giving tzedakah to atone for his sins, though, there is no limit - just as a person will spend all he has in search of a cure for his bodily illnesses, so too with spiritual ones.
According to here:
[The twenty percent rule] applies only to someone whose income does not provide generously for his family. Someone who has a job or business that provides adequately for his family is permitted to give everything above his needs to tzedakah even if it is more than twenty percent of his income or his holdings.
And from here:
Apparently, when Rav Yosef Sholom Elyashiv was asked he replied that he permits it. When he is summoned to the Heavenly Court, he added, he will present his Halachic reasoning for this ruling. If it is accepted, well and good, and if not – he is willing to accept upon himself the consequences!
I strongly associate with the latter view.
I heard from someone who asked Rav Yaakov Weinberg of Ner Israel this question. He responded that the reason it is forbidden to give away more than 20% is because the purpose of money is not only to spend it. There is a purpose in having money. By human nature, having money and possessions gives someone stature and therefore clout. Rav Weinberg said that it is important to have people of stature who are looked up to as being important who can be leaders and set examples that will be followed. (This was in response to the questioners formulation that 25% for multimillionaires is seemingly not going to create a worry of "needing from others" down the road any more than 10% would for someone less wealthy, which is the reason given in the Gemara.)
Be a chassid and as much as you are able to give of your own to others.