I didn’t give Matanos L’Evyonim on Purim. How do I make it up now?
You can’t, though Tzedaka is a good thing always. The Rebbe encourages continuing the themes of Mishloach Manos and Matonos L’Eveyonim even after Purim, though the actual fulfillment of the Mitzvah is only on Purim.
סה״ש תש״נ ח״א ע׳ 351.