Kavod Harav, I said that I will take off a chomesh (for maaser) if Hashem gives me a certain yeshua, which He did. However, due to corona, I am not getting paid as much as I was before and I don’t want to give chomesh. What should I do? Is it permitted to be matir neder (annul an oath) even though we usually do not nullify vows of tzedeka? Is it necessary since I don’t intend to permanently cancel, but just for the duration of corona when I am not able to make as much as before? 
A. On question 881 regarding if one forgot to say beli neder when donating for a mi sheberach or telling that he will do any mitzva, we wrote: “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.”
It would then be better to do hatoras neder, in case later on he forgets or can’t comply with the neder.
Rabbi A. Bartfeld as advised by Horav Shlomo and Horav Aharon Miller Shlit’a