If he promised to help, he will not go back on his promise.

如果他答应帮忙，他就不会食言。

《新英汉大辞典》

You can't go back on your words once you make a promise on something!

我们不能相信他，他这人总是说话不算数！

youdao

Cultivation of the charm of a woman is the woman, her punctuality, compliance with the promise, even if never to go back on the difficulties encountered;

魅力女人是有修养的女人，她守时，遵守诺言，即使遇到困难也从不食言；

youdao

Now the government is threatening to go back on its promise.

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And we can't go back on that promise.

NPR: Patrick Murphy: From Iraq War to Congress

Many workers, already protected by generous pension provision, with retirement possible at 50 for train drivers, agreed to go back to work on the promise of yet another bonus and pay rise.

ECONOMIST: The French Socialist Party lurches left