Is there an actual source for the widespread custom to give enormous amounts of tzedakah specifically on Purim?
Thank you for your question.
There are a few sources for the custom to give lots of money for tzedakah on Purim. However, the most direct source for this is what the Mishna Berura says that although we have a number of mitzvas to do on Purim. We give out lots of Mishloach Manos, we make a festive meal, with lots of wine and delicacies, however says the Mishna Berura (694-3) that the mitzva we should spend the most money on is matanos levyonim. The reason being “there is no greater joy and beauty, than bringing joy to the hearts of orphans and widows, similar to the shechina on which it says “to liven the spirit of the downtrodden, and the hearts of the oppressed”. We all know how much money is spent on a lavish Purim seuda, and the tens of expensive mishloach manos. Nevertheless, the real place for us to spend our money is by giving a nice amount of tzedakah on Purim.