go back on your 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!
See also: back, on, word

go ˌback on your ˈword

not do what you have promised; break a promise: He said he wouldn’t charge more than €150, but he went back on his word and gave me an invoice for €200.
See also: back, on, word