This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
If the consideration appears in the written contract as a contractual term thereof, an oral agreement whereby an additional or other consideration is provided for violates the parol evidence rule and is unenforceable.1 Thus in a contract for the sale of land, if it specifies the amount which the vendee agrees to pay, an oral contract whereby he agrees to pay more is unenforceable.2 So in other contracts of sale, where the amount to be paid is agreed upon as a contractual term, oral contracts for the assumption of the vendor's debts in addition to the amounts specified in the contract, are unenforceable.3 So where A agreed to sell B quinine at fifty-nine cents an ounce, an oral agreement whereby A agreed to advance the price to sixty-one cents per ounce, and to send out trade circulars announcing such advance is unenforceable.4 Where an injured employee signs a release of damages in consideration of payment to him of twenty-five dollars, and all the expenses of physicians and hospital, an oral agreement that the twenty-five dollars was a mere gratuity, and that accordingly the only consideration was the payment of the expenses for physicians and hospital is unenforceable.5 So in an agreement for the sale of stock at a certain price per share an oral agreement that the vendee should pay only one fourth of the amount set forth in the written contract is unenforceable.6 So where a bill of sale sets forth the price to be paid for stock an oral contract to furnish such certificates and proofs of pedigree of such stock as would enable the vendee to have them registered is unenforceable.7 So where a written contract shows that the consideration was to be determined in the future according to the amount of work done, but was "not to exceed five hundred dollars per week," an oral contract fixing the amount of compensation is unenforceable.8 So where a contract and conveyance of a right of way shows the consideration, an oral contract for an under-crossing, as an additional consideration, is unenforceable.9 And so where A sold certain patents to B, and guaranteed their validity, and B was to pay A certain royalties thereon, a subsequent written contract whereby, in lieu of such royalties, A is to receive a lump sum cannot be shown to rest in part upon an oral contract whereby B releasee A from his contract guaranteeing the validity of such patents.10 So oral evidence cannot be considered to show a lower rent than that specified in a lease.11
1 Schneider v. Turner, 130 111. 28; 6 L. R. A. 164; Indianapolis Union Ry. v. Houlihan, 157 Ind. 494; 60 N. E. 943 ; City of Paris v. Lilleston (Ky.), 60 S. W. 919; Cassard v. McGlannan, 88 Md. 168; 40 Atl. 711; Grier v. Ins. Co., 132 N. C. 542; 44 S. E. 28; Kahn v. Kahn, 94 Tex. 114; 58 S. W. 825; Buena Vista Co. v. Billmyer, 48 W. Va. 382; 37 S. E. 583.
2 Trice v. Yeoman, 60 Kan. 742; 57 Pac. 955.
3 Thompson v. Bryant, 75 Miss. 12; 21 So. 655; Walter v. Dearing (Tex. Civ. App.), 65 S. W. 380.
4Engelhorn v. Reitlinger, 122 N. Y. 76; 9 L. R. A. 548; 25 N. E. 297.
5 Indianapolis Union R. R. v. Houlihan, 157 Ind. 494; 54 L. R. A. 787; 60 N. E. 943.
6 Libby v. Spring & Land Co., 67 N. H. 587; 32 Atl. 772.