To understand the connection between the מצווה of צדקה and specifically the Yom Tov of Purim, we need to to understand the general מצווה of צדקה. The Rambam in the beginning of the tenth chapter of הלכות מתנות עניים exhorts us to be more careful in the degree of our observance of the מצווה of צדקה than all other positive commands. This is because צדקה—vis a vis the propensity to give—is an identifying trait of Avraham Avinu. As the Pasuk in בראשית יח יט states:
כִּי יְדַעְתִּיו לְמַעַן אֲשֶׁר יְצַוֶּה אֶת בָּנָיו וְאֶת בֵּיתוֹ אַחֲרָיו וְשָׁמְרוּ דֶּרֶךְ ד’ לַעֲשׂוֹת צְדָקָה וּמִשְׁפָּט לְמַעַן הָבִיא ד’ עַל אַבְרָהָם אֵת אֲשֶׁר דִּבֶּר עָלָיו
“For I have known him, that he commands his children, and his house after him and they have kept the way of Hashem, to do Tzedaka and judgment, that Hashem may bring on Abraham that which He had spoken concerning him.”
The Rambam’s statement is extraordinary. We could be forgiven for thinking that the defining characteristics of a Jew is seen through the Mezuza on their door, or through their observance of the Mitzvah of Tzitzis, Tefillin, Kashrus, and similar. Giving Tzedaka, however, is a universal humanistic value that is not just the domain of Jews, so why did the Rambam specifically choose Tzedaka as the characterising hallmark of the Jew?
In the second סעיף the Rambam goes on to say that if someone exhibits cruel or uncaring characteristics, then we might suspect that persons יחוס (intrinsic DNA) as a Jew. As is well known (and more recently popularised in Avraham Frieds’ ביישנים song) , the Gemara (יבמות מט) says
“סימני ישראל האומה הקדושה: בישנים, רחמנים וגומלי חסדים”
“The signs of the holy nation of Israel are that they are milquetoast, pitying, and bestowers of kindness.”
The Rambam stresses that all Jews are siblings and if one sibling does not look out for another, who will?
In this regard, the Megillah states the inspiring words:
וירא המן–כי אין מרדכי, כרע ומשתחוה לו; וימלא המן, חמה. ויבז בעיניו, לשלח יד במרדכי לבדו–כי הגידו לו, את עם מרדכי; ויבקש המן, להשמיד את כל היהודים
that Mordechai refused to cow tow to Haman. Haman was enraged, and whilst he would ordinarily have materialised his rage solely through punishing Mordechai, once he realised that (consistent with the Rambam’s view) Mordechai was an iconic symbol conjoined with Jews, כי הגידו לו, את עם מרדכי as a unified whole, then ויבז בעיניו he despised Mordechai more, and decided that he’d take it out on all Jews—להשמיד את כל היהודים.
This is also what Mordechai reminded Esther of when he said:
״אל תדמי בנפשך להמלט בית המלך מכל היהודים כי אם החרש תחרישי בעת הזאת ריוח והצלה יעמוד ליהודים ממקום אחר ואת ובית אביך תאבדו
“Do not imagine that you in the king’s palace can escape any more than all the Jews. For if you remain silent at this time, relief and deliverance will arise for the Jews from another place and you and your father’s house will perish.’’
Mordechai was stressing to Esther that Jews are responsible for each other; they are a singleton. It cannot be that a Jew’s fate is detached from other Jews anymore than a Jew can absolve themselves from their responsibility to another Jew. This theme, of the unity of the proverbial Jewish body is also channeled to a prospective גר. After detaching themselves from any prior nation or peoplehood, the גר is informed that they will now face anti-Semitism in the same way as any born Jew. The proverbial Jewish body has absorbed the גר and they are now a vital component.
This message—the importance of perpetuating the unity of the Jewish nation—as echoed by the מגילה and מרדכי and המן himself, motivated the Rabbinic enactment of the מצווה of מתנות לאביונים. Enveloped with this theme, the מצווה is not simply one of צדקה but is now the materialistic expression of a unifying act which by its purpose is consistent with not checking the credentials of the person one is giving the money to. On the contrary, the body is in tact, knows no difference, discerns no hierarchy and for this—the theme of כל ישראל עריבים זה לזה and אחדות—it is consistent to give to any Jew and not to question their bona fides.