Q. What if I forgot to say beli neder when donating for a mi sheberach or telling that I will do any mitzva, does (Kol Nidre or Hatoras Nedorim declaration of annulment) help?

A. It is always better to say b’li neder. However, Horav Shlomo Miller’s Shlit”a opinion is that when someone did not have in mind to make a neder at all and he also did not remember then the declarations of annulment made in Kol Nidrei or Hatoras Nedarim, in need he may rely on them to avoid additional Hatoras Nedarim
However, if he did use an expression of neder when he donated tzedaka, or he answered amen to “ba’abur shenodar” said aloud by the gabay reciting a “mi sheberach” for him, it does become a neder if he omitted adding then b’li neder. However, he does not have to repeat b’li neder after answering omen on the “mi sheberach” if the gabay omitted saying b’li neder in turn, as the gabay is accepting the amount and the condition of the contributor.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a.