Is there a halacha in hilchos tzedak that says, give all you can to the 1st person who asks and then have nothing for the next person.
My personal practice is to give 3 or 4 dollars to whoever comes to the door knowing that a number of people will come. If it is not good enough for the person, i tell the collector, this is what i have, if you dont want it, i’ll give it to someone else.