go back on one's word
and go back on one's promise; go back on one's pledgeto break a promise that one has made. I hate to go back on my word, but I won't pay you $100 afterall. Going back on your promise makes you a liar.
pledge something
to someone to promise something to someone. I pledged one hundred dollars to Ralph for his cause. We pledged a lot of money to our favorite charity.
take the pledge
Fig. to promise to abstain from drinking alcohol. I'm not ready to take the pledge yet, but I will cut down. My aunt tried to get me to take the pledge.