To Gain and to Give: Hilchot Tzedakah, Day 13
The first time one takes off ma’aser, it is given from the principle. Subsequently, ma’aser is given from one’s earnings. For example, Sarah has $30,000 and wants to start giving ma’aser. She takes one-tenth of her wealth, i.e., $3,000 and gives it to tzedakah. The next year, Sarah made a net profit of $15,000. Sarah now deducts a tenth from her profit, and gives $1,500 to tzedakah (to be continued).
ש"ך ס' רמט סק"ב