67:3 One should be very careful not to make any vows, even not to vow to give charity. If one has something that he wishes to donate, he should simply do so. If he doesn’t, then he should wait until he does, and then donate it. In no case should he make a vow about it. If pledges to charity are being taken and he wants to pledge, he should specify that he is pledging “bli neder” – that is, without it constituting a vow. Similarly, when pledging charity in memory of deceased relatives (e.g., at Yizkor), one should specify “bli neder,” that it is not a vow. A person is permitted, however, to make a vow in a time of trouble.
67:4 If a person wants to establish a time for Torah study or to do a certain mitzvah and he is afraid that he will neglect it, or if a person is afraid that his yetzer hara (“evil inclination”) will tempt him into doing something wrong, he is permitted to strengthen his resolve by making a vow or an oath. The Talmud in Nedarim (8a) says, “How do we know that one may make an oath to fulfill a mitzvah in order to strengthen his resolve even though he is already bound by the oath made at Mount Sinai? We learn it from Psalms 119:106, which says, ‘I have sworn and I will fulfill to observe your righteous laws.’” Even if one did not state this commitment in the form of an oath or a vow, but he merely made a general statement, it is still considered a vow and it must be fulfilled. Therefore, one must be careful to specify “bli neder” when saying that he will perform a mitzvah. It is advisable to accustom ourselves to saying “bli neder” even when promising to do voluntary things as a precaution against accidentally violating the prohibitions regarding vows.