It is brought down in halacha (Orach Chaim 656) that a person is not obligated to spend more than 20% of his money on performing a positive mitzvah. Is the 20% calculated according to his total bank account, or is it calculated like ma'aser kesafim is, according to his yearly income. For example, if a certain mitzvah would cost more than 20% of the man's income for that year, but it would be less than 20% of his total bank account that he has saved up from forever, would he be obligated to perform that mitzvah?
It can only mean: "Give no more than 20% of what you earn as you earn it".
The alternative doesn't make sense. Suppose that, starting from 0, you earn $100. You give $20 to charity, spend $10 for yourself, and put the remaining $70 in the bank. Now, if you "tax" that $70 at 20%, then what remains at 20%, and keep going, before too long you will have next to nothing left.