We agreed beforehand - no one is allowed to back out.
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history
The phrase appears in Feng Menglong's late-Ming collection 醒世恒言 ('Stories to Awaken the World'): 他有言在先,你今日不须惊怕 - 'he gave prior warning, so you needn't be afraid today.' Used today to invoke a previously-stated condition or warning.
usage
Often appears at the start of a sentence to preface a condition you're about to enforce: 有言在先, ... = 'Just so we're clear...' The implication is the listener can't claim ignorance later.