If I see a man going around my shul collecting tzedakah and I have in mind to give him some tzedakah (is there a difference if I think of a certain amount) and then he leaves; do I have to find him to give him the tzedakah because I had in mind to give it to him?
If you see someone collecting tzedakah and you only had a fleeting thought to give him, and you can’t find him later, you don’t have to an obligation to give him. However if you firmly decided to give him you have to give the money however if you can’t find him you can give the money to a different poor person.
As a matter of habit, in order to avoid such issues in the future, that whenever you will think of giving to a person collecting tzedakah, that you only mean to give to give it to him if it indeed reaches him. But aside from that
Derech Emuna Chap 8 footnote 8,19 from the Chazon Ish. Kol Torah 39 responsa 24 from R’ S.Z. Auerbach zt”l.