To Gain and to Give: Hilchot Tzedakah, Day 21

If one realized that they previously gave ma’aser in excess of their obligation, they are permitted to deduct the excess amount from their future ma’aser obligation. Preferably, when one begins taking ma’aser for the first time they should verbally state that if they give more ma’aser than required, they may deduct that amount from their future ma’aser calculation.

אהבת חסד פי"ח ס"ה