The Rishonim (Medieval Halachic scholars) address the question regarding the status of a commitment to charity made in one’s mind, without being verbalized. If a person made a decision to give a certain amount of money to charity, but he did not verbally express this commitment, does this non-verbal pledge have the status of a Neder (vow), such that one is obligated to fulfill the pledge?
The Rosh (Rabbenu Asher Ben Yehiel, Germany-Spain, 1250-1327), in one of his responsa, writes that a pledge to charity is binding only if it is verbalized. A commitment made in one’s mind, without being stated verbally, is not binding.
However, in his commentary to the Talmud, the Rosh takes the opposite view, stating that even non-verbal pledges are binding and must be fulfilled. The Rosh cites a verse in Dibreh Hayamim – "Kol Nedib Leb Olot" – referring to a voluntary commitment made in one’s heart to bring Olot (burnt offerings) to the Bet Ha’mikdash. Based on this verse, the Rosh rules that if a person resolves in his mind to give money to charity, this commitment is Halachically binding and he must fulfill the pledge.
The Rama (Rabbi Moshe Isserles of Cracow, 1525-1572), in Hoshen Mishpat (212:8), follows the stringent ruling, and thus for Ashkenazim, it is clear that if one makes even a non-verbal pledge, it is binding. It should be noted that we refer only to a resolute decision to give charity. If one merely contemplated in his mind giving charity, or considered the possibility, this is certainly not a binding pledge. We deal here only with a firm, unambiguous decision to give charity.
Maran, however, in Shulhan Aruch, cites both views. He first writes, "Yesh Omerim" ("There are those who say") that a non-verbal pledge is binding, and then writes, "Yesh Mi She’omer" ("There is someone who says") that it is not binding. There is a well-known rule that in a case of "Yesh Va’yesh," where Maran cites two different opinions as "Yesh Omerim," we follow the second opinion cited. Seemingly, then, in this instance, we should follow the lenient position, that a vow taken in one’s mind is not Halachically binding. However, Hacham Ben Sion Abba Shaul (Israel, 1923-1998) claimed that this rule applies only if Maran uses the precise phrase "Yesh Omerim" in reference to both views. However, if he cites one view as "Yesh Omerim" and the other with the phrase "Yesh Mi She’omer," then he sides with the "Yesh Omerim," which is the majority position. Hence, with regard to the Halacha of a non-verbal pledge, Maran follows the stringent view, which he introduces as the "Yesh Omerim." Hacham Ben Sion further notes that given the especially grave consequences of violating vows, one who makes a non-verbal pledge should ensure to fulfill the pledge, in accordance with the stringent opinion.
Our Sages teach that as a general rule it is preferable not to make formal pledges, in order to avoid the risk of violating a Neder. And thus certainly when it comes to non-verbal pledges, the status of which is subject to dispute, one should preferably not make formal pledges in his mind in order to avoid this Halachic uncertainty.
Summary: Formal pledges to give charity are binding even if they are made in one’s mind, without being verbalized.