There are numerous exceptions to the parol evidence rule. One is that extrinsic evidence is admissible to prove an unfulfilled condition precedent. The classic decision cited in that regard if Pym v. Campbell, 119 E.R. 903. The exception in question is that extrinsic evidence is admissible to show that a document which, on its face, is an agreement between the parties is not in fact an agreement.
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