One of the most common questions involving tzedaka concerns the commitment to give. It is a Torah prohibition to make a vow (a neder) and then fail to fulfill that vow. If one vows to give a sum to tzedaka one must do so and one is not allowed to delay giving it. The Shulchan Aruch (Yoreh Deah 203:4) recommends not making vows to give tzedaka since it is such a serious offence to fail to fulfill a vow. On the other hand, the Chofetz Chaim (in Ahavat Chessed 2:16, in the notes) maintains it is actually a separate mitzvah to make a vow to give tzedaka.
The normal way to commit to tzedaka has therefore become that one says “bli neder” (without a vow) when making any commitment to give. One must actually say this out loud – it is not enough to think it. This is required because if one does not say it we assume that a commitment to give tzedaka some hold it is indeed a vow and is fully binding. For example, when one receives a “Mi Sheberach” (the brachah that one receives after getting an aliyah to the Torah), one should make sure to say “bli neder” when committing a certain amount as a donation to the shul or as a merit for an iluy neshama.
In fact, there is a custom to make commitments (without saying bli neder) to give tzedaka in several situations:
1) On Shabbat and Yom Tov in shul, since everyone is gathered together and it is a good opportunity to raise funds for the needy.
2) In times of danger, as we find that Yaakov Avinu took a neder when in fear of fighting. The Shulchan Aruch (Yoreh Deah 203) rules that it is proper to vow to give something to tzedaka in times of danger, as does the Ben Ish Chai in Parshat Re’eh Shana Shelishit. Examples of times of danger can be found in the stories with Rabbi Meir in the gemara Avodah Zarah 18a-b.
3) One may also take a vow in order to encourage others to give, although one must of course still fulfill the vow and give what he committed to give.