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 contract is one which is in writing, but is not required by law to be in writing, or to be proved by writing, such contract may be modified by a subsequent oral agreement, if such agreement contains within itself the elements of a valid contract.1 The parties may abandon or modify a written contract by an implied agreement which is to be inferred from their acts and conduct.2 Where A agreed in writing to construct a boiler and engine in B's barge within a given time, such contract may be subsequently modified by an oral agreement between A and B, fixing the time at which such barge is to be delivered to A.3 A written contract for the sale of a machine, to be paid for before delivery, may subsequently be modified by the oral agreement of the parties that the vendee shall have the right to make trial of such machine, and to return it if not satisfactory, followed by delivery to vendee of the machine without demanding payment therefor.4 A written agreement by A, an attorney, to manage litigation for B for two and one-half per cent. of the amount recovered, if the allowance by the viewers is final, and five per cent. of the recovery, if the case is appealed to the court for trial, may be modified after verdict is obtained in court by an agreement that if A prevents a new trial, and prevents a reduction of the verdict, he shall have everything above a certain sum; that if the verdict is reduced below such sum, and above another sum, A is to receive nothing; and that if the verdict is reduced below such latter sum, B is to take a new trial, and pay A ten per cent. of the recovery upon the second trial.5
5 McKenzie v. Stewart. 196 Ala. 241, 72 So. 109; Manlove v. Lemmon, 272 111. 120, 111 N. E. 739; Murray v. Boyd, 165 Ky. 625, 177 S. W. 468.
1 United States. Teal v. Bilby, 123 U. S. 572, 31 L. ed. 263; Hull v. Pitrat. 45 Fed. 94.
Alabama. Elliott v. Howison, 146 Ala. 568, 40 So. 1018.
District of Columbia. Cumberland, etc., Co. v. Wheatley. 9 D. C. App. 334.
Florida. Robinson v. Hyer, 35 Fla. 544, 17 So. 745; Gunby v. Drew. 45 Fla. 350, 34 So. 305.
Iowa. Lamb's Estate v. Morrow, 140 Ia. 89, 18 L. R. A. (N.S.) 226, 117 N. W. 1118; Lefebure v. Lord. - Ia. - , 167 N. W. 651.
Kentucky. John King Co. v. Louisville & N. R. Co., 131 Ky. 46, 114 S. W. 308 [rehearing denied, 116 S. W. 1201].
Louisiana. Levy v. Levy, 139 La. 274, 71 So. 507.
Maine. Hilton v. Hanson. 101 Me. 21, 62 Atl. 797.
Michigan. Smith v. Kelley, 115 Mich. 411, 73 N. W. 385; Grand Traverse Fruit & Produce Exchange v. Thomas Canning Co., 200 Mich. 95, 166 N. W
Minnesota. Van Santvoord v. Smith 79 Minn. 316, 82 X. W. 642; Hagstrom v. McDougall, 131 Minn. 389, 155 X. W. 391.
Nebraska. Bryant v. Thesing. 46 Neb. 244, 64 X. W. 967; Strahl v. Grocer Co. (Neb.). 98 X. \V. 1043.
New York. Solomon v. Vallette. 152 X. Y. 147,46 X. E. 324.
North Carolina. May v. Getty. 140 X. Car. 310, 53 S. E. 75; Acme Manufacturing Co. v. McCormiek, 175 X. Car. 277, 95 S. E. 555.
North Dakota. Quinlivan v. Denn-tedt Land Co., - X. D. - . 168 X W. 51.
Oregon. Moll v. Roth Co.. 77 Or. 593, 152 Pac. 235.
Pennsylvania. Moore v. Carter, 146 Pa. St. 492, 23 Atl. 243; Neffs- Estate. 185 Pa. St. 98, 39 Atl. 830; Beatty v. Larzelere, 194 Pa. St. 605, 45 Atl. 653: Achenbach v. Stoddard, 253 Pa. St. 338, 98 Atl. 604; Robert Grace Contracting Co. v. Norfolk & W. Ry. Co.. 259 Pa. St. 241, 102 Atl. 956 (obiter).
Utah. Morgan v. Child. 47 Utah 417, 155 Pac. 451.
Vermont. Davenport v. Crowell. 79 Vt. 419, 65 Atl. 557.
Virginia. J. P. Houck Tanning Co. v. Clinedinst, 118 Va. 131, 86 S. E. 851.
Washington. Dignan v. Spurr. 3 Wash. 309, 28 Pac. 529; Wallace v. Babcock, 93 Wash. 392. 160 Pac. 1041.
If a written contract is modified in part by a subsequent oral agreement, it is said, in some jurisdictions, to be reduced entirely to an oral contract.6
A written contract which has been signed by all the parties thereto, may be modified by a subsequent written contract, agreed to by all the parties, but signed only by the party who surrenders a right or advantage which the original contract gave him.7
Wisconsin. Wisconsin Sulphite Fibre Co. v. D. K. Jeffris Lumber Co., 13*2 Wis. 1, 111 N. W. 237.
"It i well settled that parties who have undertaken contractual obligations by an agreement in writing may nevertheless enter into a new parol agreement creating obligations separate from the old ones and at variance with them, and such new agreement will be binding, unless the contract is one required by the statute to be in writing." Quinlivan v. Dennstedt Land Co., - N. D. - . 168 N. W. 51.
2Hilis v. McMunn. 232 111. 488, 83 N. E. 963; May v. Getty, 140 N. Car.
310, 53 S. E. 75; Davenport v. Crowell, 79 Vt. 419, 65 Atl. 557.
3 Manistee Iron Works Co. v. Lumber Co., 92 Wis. 21, 66 N. W. 863.
4 McGregor v. Register Co., 86 Ga. 439, 12 S. E. 683.
5 Beatty v. Larzelere, 194 Pa. St. 605, 45 Atl. 653.
6 Malone v. R. R., 157 Pa. St. 430, 27 Atl. 756.
7 Bray v. Loomer, 61 Conn. 456, 23 Atl. 831.
If the written contract provides for payment or tender of money, oral evidence showing that such provision has been modified or discharged is admissible.8 As between principal and agent, the agent may show by oral evidence that his written authority,9 such as his power of attorney,10 was modified. If a contract of partnership is in writing, the partners may show as between themselves that such written contract was modified by oral agreement after it was made.11 Even if a contract of indorsement is regarded as being in writing, oral evidence of a subsequent agreement by which the indorser agreed to collect such instrument as agent for the indorsee, is admissible.12