go back on one's words

go back on one's words ( to not be faithful or loyal to one's word or an agreement; break a promise) — идти на попятный; отказываться от уже принятого решения

Example 1: Patrick went back on his word when he refused to marry Karen in spite of his earlier promise. (Словарь американских идиом)

Example 2: The company directors went back on their word to give the employees a salary increase.

[пойти на попятный]

opp.:
be as good as one's word — сдержать слово; быть верным своему слову; не обмануть
keep one's word / promise

Example 1: Where did you get that wine barrel planter?" Jean asked. Linda said, "I worked for Mr. Marcus and he promised that if I completed the job by last Tuesday, he would give me the planter as a bonus. I finished last Tuesday and he was as good as his word." (nglish-test.net)