In the Shulchan Aruch it says that someone who is approached by a poor person asking for a donation is not allowed to let him go empty handed. He needs to oblige him. To this Reb Ya-akov zl stipulated that it applies only when being approached by the poor person himself. It does not, however, apply when someone is approached by someone who collects in behalf of a poor person or poor people. Much less, he added, does it apply when someone is approached via a mailed solicitation.
It happened that someone announced in shul that he was authorized by the administration of the shul to appeal to the congregation for funds for the benefit of a certain cause. The members of the congregation made pledges in response to the appeal. It transpired, however, that he had made his claim falsely and had never been so authorized. Reb Ya-akov zl ruled that the responders were not required to honor their pledges, and that their pledges were considered to have been made in error and were, therefore, not binding.
(Adapted from Bemechitzas Rabeinu)