Verbal evidence is inadmissible to vary or contradict the terms of a written agreement.
In Wheeler, Kelly & Hagny v Curtis, cited at 158 Kan 312, the Supreme Court of Kansas, in 1944, adopted these words, with extensive quoting from other cases:
"Parol evidence of an agreement consisting of mere oral promises made previously or concurrently with the execution of a written contract of sale of land is inadmissible to charge the vendee with the payment of more than the expressed consideration, when the amount to be paid plainly appears from the face of the instrument.
"As a general rule all prior oral negotiations are deemed to be merged in a written agreement, and the terms of such agreement cannot be contradicted, altered, added to or varied by parol proof.
"An exception to the rule is a unilateral admission, such as an ordinary receipt, or the mere acknowledgment of the receipt of purchase-money in a conveyance, which ordinarily is not conclusive upon the parties as to the consideration stated; but when it appears from the writing that the statement of consideration is contractual, and not merely matter of recital, it is not open to contradiction by oral proof.
"It has been well said that 'the rule permitting the true consideration of written contracts to be inquired into by parol evidence does not apply where the statement in the contract as to the consideration is more than a mere receipt or acknowledgment of payment, and is of a contractual nature.
"It is a general rule that parol or extrinsic evidence is not admissible to add to, subtract from, vary, or contradict judicial or official records or documents, or written instruments which dispose of property, or are contractual in nature and which are valid, complete, unambiguous, and unaffected by accident or mistake. This rule, which is known as the parol evidence rule, is one of substantive law and not merely one of evidence; and it obtains in equity as well as at law.
"The parol evidence rule applies to written contracts for the sale or exchange of reality or personalty, and to such provisions as those relating to price, time and mode of payment, and time and place of delivery.
"The statement in a contract of sale as to the price paid or to be paid by the purchaser is an essential part of the contract, and hence it cannot be varied or contradicted by parol."
In Canada, where the rule is also applicable, the Alberta Court of Appeal, in Gainers Inc. v. Pocklington Financial Corporation, 2000 ABCA 151, published at canlii.org/en/ab/abca/doc/2000/2000abca151/2000abca151.html, described it as follows:
"When the deal is complete in the written contracts...., other evidence (parol evidence) is inadmissible to vary or contradict a clear written contract."