If you promised to give a $500. a month to a given charity as long as you had the money, and later your expenses go up such that the only way to give the money to the charity is to not feed your family, are you still obligated to give the money?
Generally, a promise must be kept (and a promise to give charity is effective when taken using lesser methods than a standard promise), and for that reason we avoid promising. There are ways to annul promises: one must go to someone who knows the rules of such annulments (so go to your rabbi!), and he will determine whether it can be annulled. In brief, a promise can often be annulled if a circumstance has since arisen that was not foreseen at the time and that, had it been foreseen, would have meant the promise wouldn't have been taken.