Can I give ma'aser on money that I didn't earn yet but I know I will be earning in the future?
According to the Chofetz Chaim (Ahavas Chessed 18:2), this is subject to a dispute among the poskim. In order to avoid this problem, the Chofetz Chaim recommends that when one starts to distribute maaser, he should explicitly say that he is accepting this practice on the condition that he can distribute tzedakah before earning the money.
The Tzedakah U'Mishpat (5:11) explains the above applies only if one gave the money to tzedakah without any preconditions and only later decided to have it be considered maaser. However, if he explicitly gave the tzedakah with the intention of it being considered maaser on future earnings, one may deduct it from his future obligation.