One reason, according to the Rashba, as to why we do not recite a blessing over the giving of Mattanoth LaEbyonim (gifts to the poor) and Mishlowah Manoth (sending portions of food to our friends) on Purim, is because perhaps they will not accept them from us
It is far better to increase the giving of Mattanoth LaEbyonim than to increase the food during the Se'uddah (festive meal) or the amount of Mishlowah Manoth one sends to one's friends, because there is no greater happiness than when we make the hearts of those who are less fortunate than we are, happy.
In places where the custom is for members of the community to participate in giving gifts to the Hazzan before Purim, the money they give cannot be considered as Mattanoth LaEbyonim. One's Ma'aser money (tithes) cannot be used for Mattanoth LaEbyonim but must be in addition to one's Ma'aser. However, if one wishes to give more than is required, the additional portion may come from the Ma'aser. The money for the Mahasith Hasheqel (half Sheqel) must also be in addition to one's Ma'aser.
(See Kaf Hahayim 694, Oth 4, 5, 14 & 16.)
