一言为定

lit. to fix the tone with a single hammer blow; fig. to make the final decision

to say one and mean just that (idiom); to keep one's word

lit. once said, a team of horses cannot unsay it (idiom); a promise must be kept

each word carries weight; a promise must be kept (idiom)

to keep one's promise; to mean what one says

to speak and keep one's promise (idiom); as good as one's word