一诺千金 One promise, a thousand gold. A promise of the highest worth. It goes back to a saying of Chu recorded by Sima Qian: “Better to get a hundred gold than one promise from Ji Bu.”
一言既出，驷马难追 Once a word is out, a team of four horses cannot catch it.
一言为定 One word settles it. A word once given is not changed or taken back.