go back on one's word 出尔反尔

中国人常说“一诺千金”，意思是“A promise is weightier than one thousand bars of gold”。做出承诺就应该去兑现，不应该“出尔反尔”。

“出尔反尔”，汉语成语，多指自己说了或做了，后来又反悔。比喻言行自相矛盾，反复无常，说话不算数（promise and then deny in succession;be inconsistent in one's words）。可以翻译为“go back on one's word”或“ renege on one's word”，表示“to break a promise that one has made；to not do something one said one would do”。

例句：

你答应支持我们的项目，现在却出尔反尔！

You made a commitment to support our project, and now you've gone back on your word!

我生气是因为你出尔反尔！

I am mad at you because you reneged on your promise!

我不想出尔反尔，但是我不会付你100美元的。

I hate to go back on my word, but I won't pay you $100.