go back on word
go back on (one's) word
To not do something one said one would do; to renege on something. People are beginning to worry that the president is going to go back on his word and not lower taxes for lower-income families after all. You made a commitment to support our project, and now you've gone back on your word!
go back on one's wordand go back on one's promise; go back on one's pledge
to 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.