If someone intends in his mind to give a certain amount to tzadakah, but later changes his minds and wants to give less for various reasons? Perhaps there are more requests for money that was not prevalent before,etc.Or he just changed his mind. Is he breaking a neder? What about with other matters as well, if someone changes his mind? What’s a good advice in this matter.
It is in general permitted for a person to change his mind on financial matters, and a person has the right to act how he wishes with his money.
Of course, a person should act honestly and in good faith, and where changing one’s mind implies acting in bad faith (for instance, backing out of a contract after all terms have been arranged), the Sages see the action in a negative light (some understand this to imply a full halachic prohibition).
However, for tzedakah purposes, if a person has made a resolute decision to give a certain amount of money to tzedakah, there is concern that this is considered a neder, and the amount should be given. However, this only applies to a resolute decision, and not to a mere thought.
If a person made up his mind to give to a particular cause, and then wishes to change his mind and give to another cause, this is permitted, provided the money is given to some charitable cause.
For sources and more details on this matter, please see here.