How is it possible that you would only be able to do Matanos L'Evyonim with food? (you have money)
Rav Yitzchok Silverstein writes that if Purim comes out on a Friday, and you were Mekabel Shabbos early and then you realized that you forgot to give Matanos L'Evyonim, at that point you can still do the Mitzva by giving food to a poor person, although you would be unable to give money to him.
In Yerushalayim or other places that have Purim Mesulash when Purim comes out on Shabbos, Matanos L'Evyonim is given on Friday the same day you do Megila in such a case. If someone forgot to give it on Friday and remembers during Shabbos, which is 15 Adar, at that point he can still do the Mitzva by giving food to a poor person, although he would be unable to give money to him.