The Torah in Parashat Ki-Teseh (Debarim 23:22-23) issues two commands relevant to the fulfillment of pledges. First, it commands that if a person makes a pledge to G-d, “Lo Te’aher Le’shalemo” – it is forbidden for him to delay the fulfillment of the pledge. The Torah then issues an affirmative command to follow though on one’s pledges: “Mosa Sefatecha Tishmor Ve’asita” (“You shall keep and perform that which leaves your mouth”). Fulfilling pledges thus involves two separate commands: a prohibition against delaying the fulfillment of a vow, and an affirmative command requiring the prompt fulfillment of the vow.
The Sefer Ha’hinuch brings two different opinions regarding these two commands. According to one view, the prohibition of “Bal Te’aher” (delaying the fulfillment of a pledge) forbids delaying the fulfillment of a pledge for the period of three Regalim (the pilgrimage festivals – Pesach, Shabuot and Sukkot). Meaning, if, for example, somebody vowed to offer a voluntary sacrifice in the Bet Ha’mikdash, he is in violation of “Bal Te’aher” only if three Regalim pass without him fulfilling the vow. The affirmative command to fulfill a pledge, however, is transgressed immediately, once one has the opportunity to fulfill the pledge and is unable to do so. Thus, if a person pledges money to charity, and he has the money available, then he is required to pay his pledge immediately, since there are, presumably, needy individuals who require charity, such that the pledge can be fulfilled right away.
Some Poskim, however, maintain that even the prohibition of “Bal Te’aher” is violated if one does not immediately fulfill his pledge to charity. According to this view, the period of three Regalim is relevant to the case of a pledge to offer a sacrifice, and to the mandatory charitable donations that one is obligated to give, such as various portions of one’s agricultural produce (Leket, Shicheha and Pe’a). When it comes to other forms of charity, however, in the view of these Poskim, one transgresses both the affirmative command and the prohibition of “Bal Te’aher” once he has the opportunity to fulfill his pledge but fails to do so.
This applies also to pledges made to a synagogue. A synagogue employs staff whose livelihoods depend on their salaries, and who are thus considered like Aniyim (needy individuals) with respect to this Halacha. Once a person makes a pledge to a synagogue, he must fulfill his pledge as soon as he can, as the synagogue needs funds at all times to pay its employees. One who unnecessarily delays the fulfillment of his pledge to the synagogue violates the affirmative command of “Mosa Sefatecha Tishmor Ve’asita,” and, according to some Poskim, he also violates the prohibition of “Bal Te’aher.” If one does not have the funds available to fulfill his pledge, he should pay with a credit card.
Of course, it is far easier to pledge than to pay. However, Halacha requires paying as quickly and easily as one pledges, without any unnecessary delays.
Summary: One who pledges a donation to a synagogue must fulfill his pledge as soon as he has the opportunity to do so. Unnecessarily delaying the fulfillment of a pledge to a synagogue violates the Torah’s command to promptly fulfill pledges (“Mosa Sefatecha Tishmor Ve’asita”), and, according to some Poskim, violates also a Biblical prohibition (“Bal Te’aher”). If one does not have the money available for his pledge, then he should pay with a credit card.