“The parol evidence rule, briefly stated, requires that in the absence of fraud, duress, or mutual mistake, all extrinsic evidence must be excluded if the parties have reduced their agreement to an integrated writing. Under this rule, all prior and contemporaneous negotiations or understandings of the contract are merged, once that contract is reduced in writing. Williston on Contracts, Third Edition § 631.
“As this case illustrates, application of the rule can work to create harsh results. However, the policies behind the rule compel its consistent, uniform application. Commercial stability requires that parties to a contract may rely upon its express terms without worrying that the law will allow the other party to change the terms of the agreement at a later date.” J. McDonough, Baker v. Bailey, 240 Mont. 139 (Mo. 1989)