Question: Why does Rav Elyashiv poskin that if you can’t find the person you decided to give tzedaka to you can give it to someone else?
A. The Mishna Berura (694:2:6) says that one should not switch Purim money with other tzedaka. Specifically, the Gabbaim, but a poor person can do what he wants with the money. Meaning that the money collected has to be given out for the needs of purim. The Beis Yosef, quoting Haghos Ashr”i says that money, that one thinks in his heart, he wants to give out to the poor on Purim, should not be changed. The reason is because he holds that tzedaka is like hekdesh and you have to fulfill what’s in your thoughts, even if you don’t say them out.
B. Aruch Hashulchan (Yoreh Deah 258:39) says that you can’t change only if you decisively decided in your heart to give to a certain person or place, but if you only thought you want to give this amount everyone agrees you don’t have to fulfill what you thought.
C. Rav Elyashiv, therefore, poskins that one should not decisively think to give money to a certain gabbai tzedaka if you are unsure if you will find him, rather you should think in your mind that you have no decision until it reaches the intended hands. But both the Chazon Ish and Rav Elyashiv poskin that if you decide to give money to a certain poor person and you can’t find him then you can give it to some other poor person. Why?
D. We say, Erev Rosh Hashanah by hataras nedarim, “In case I forget the conditions of this declaration and I make a vow from this day onward, from this moment I retroactively regret them and declare them that they are all totally nullified and void, without effect and without validity and they shall not take effect at all. Regarding them all, I regret them from this time and forever. ”
Answer: Rav Elyashiv says that because you can rely on the declaration made by the annul of vows on Erev Rosh Hashanah, since the vow was only in one’s thoughts, then you can give it to another poor person because you must not have decisively decided to give it to a specific person as retroactively revealed by the declaration.