Indeed, a gabbai tzedaka deserves much credit even if for all the complications that arise. We will deal with each question separately, although there are some unifying concepts.
There are two elements that make a donation binding: One involves the explicit or implicit neder (oath) to give tzedaka. The other involves the acquisition (kinyan) of the donation by or on behalf of the recipient(s). Each element has rules as to when it is binding and when a mistake renders the donation void. The Shulchan Aruch[1] rules that one may not back out of a donation, even with sh’eila,[2] once it reaches the hands of the gabbai. There is a critical question as to whether and when a tzedaka box is considered like the hands of the gabbai.[3] However, if the money was placed in the box because of a full-fledged mistake, the rules of kinyan b’taut (acquisition based on a mistake) apply, and the money may be removed and put in the intended place without problem.[4] Tzedaka is not like hekdesh,[5] and the money does not have intrinsic kedusha.[6] Therefore, it does not matter if one takes the same coins or bills he put in or different ones.
Assuming that we are talking about the same group of needy people or that the group was never clearly defined, there is no problem giving the money for similar needs. Although we find that money collected for a Purim seuda should not be switched to other purposes,[7] this halacha is interpreted by most poskim as an exception, not the rule. Certainly, when the money will be used by the same pool of poor people under similar circumstances, the gabbai may make the changes as needed.[8] If the need totally disappears, the money should be given to other recipients, preferably with comparable needs.[9]
When receiving tzedaka boxes from people and institutions, it is best to stipulate that you are planning to give the proceeds to them exclusively only if they return within a set amount of time. Even if you did not make such a stipulation, if you cannot track down the recipient, you, as gabbai, are not required to watch the money indefinitely and may transfer it to other charities of that type. If you had put in your own money without a stipulation, you should preferably do sh’eila.[10]