“A possible argument against would be that Hashem sends you a mitzvah and you should do that rather than looking for a better one…”
Sadly, you have no assurance that some guy who shows up at your door or at the beis medrash dressed in fakempt yeshivish lvush carrying a frayed letter laminated in plastic signed by some rebbelah you may never have heard of is the personification of a mitzvah sent your way by the Ebeshter. Always do your diligence and only offer funds if you have confirmed the legitimacy of the recipient (unless you simply offer a dollar bill and send him on his way) .