Abgemacht ist abgemacht.
Ein Mann, ein Wort — a man, a word.
Darauf kannst du dich verlassen — you can rely on that.
Sein Wort halten — to keep one’s word, and sein Wort brechen, to break it, which is spoken of as damage done to the word itself.
The German expressions treat the agreement as a thing that exists after the conversation ends, independent of either party’s continued attention.