In Choshen Mishpat 212:8 the Mechaber cites a difference of opinion among the early authorities regarding the obligatory nature of an unspoken, mental vow. Some authorities see this halacha as a stringency that only applies to the very serious step of consecrating (in Hebrew, to consecrate: makdish, a consecrated item: hekdesh) an item to the Mishkan or Bais Hamikdash, the Holy Temple. According to this opinion, a mental vow which does not relate to hekdesh, (e.g., a mental vow to donate money to the poor) does not need to be kept. However, other authorities rule that mental vows must be kept even in cases that do not involve hekdesh. According to this opinion, even mental vows to donate money to the poor must be kept. The Rema states that normative halacha is to follow this stringent opinion.
 
Now let us return to our case of the money collector who can no longer be located. A simple application of the Rema’s ruling tells us that since clearly a mental vow was made, that vow must be kept. Shrugging one’s shoulders and forgetting about the whole thing would not be the correct course of action.
 
This leaves us in a quandary. How feasible is it for him to keep his vow, given the fact that the charity collector can no longer be found? Actually, it turns out to be quite feasible. True, the initial intention was to give the money to the specific charity collector who had asked for the donation. However, in order to fulfill the requirements of the vow, the money can be given to a diffferent charity.
 
The later authorities explain that a vow which has not been verbalized remains a simple vow to donate a few dollars to charity in general with no specific charity having any more legal right over the money than any other charity. However, once the vow is verbalized, it is as if the money has been handed over to the charity that had been singled out in the vow, and that specific charity now has legal rights over the money. Therefore, in our case, since the vow was not verbalized and had remained a mental vow, the collector to whom those few dollars had been originally intended does not have any legal claim to the money. The person who made the vow can fulfill his obligation by donating the few dollars to a different charity. However, it is important to note that with respect to fulfilling the requirements of a vow to give to a specific type of tzedakah, it is not necessarily true that all charities are considered by the Torah to be valid replacements for all other charities.