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.
In order to operate as a discharge in whole or in part of an earlier contract, the later contract must be supported by a valuable consideration. If the original contract is still executory on both sides either in whole or in part, and the parties in forming the new contract waive, or release any liability created by the original contract such waiver or release is a consideration for the promise of the party whose liability is thus released.1 Thus the assumption of personal liability where none before existed,2 or a waiver of a right of a sub-contractor to complete the contract himself or to hire some one other than the chief contractor to complete it,3 is a consideration for a modification. If an executory contract between A and B is modified by imposing a new liability upon A without releasing him from any liability, and without imposing any additional liability upon B, such promise does not contain in itself any consideration sufficient to support A's promise to assume such new liability. Unless some consideration exists outside of the mutual promises of A and B, the new contract has no consideration and is unenforceable.4 After a written agreement is made for the sale of land, a subsequent agreement of the vendee to repay the amount received if a certain railroad was not completed in two years, is without consideration.5 A street car company, in order to obtain the consent of another company to use the tracks of the latter, agreed to reconstruct that part of the line which they desired to use, and equip it for use as an electric line. A subsequent agreement by which the company seeking to make use of such line, agreed, in addition to its former liability, to pay the costs incident to widening the track at the option of the other party, was without consideration.6 So a modification of a prior contract which relieves A from some liability imposed by such prior contract, without imposing any liability upon A in place thereof and without modifying B's liability in any way, is of no effect as a discharge of such prior contract.7 A promise made after a contract is entered into, to extend the time of performance, is without consideration and unenforceable.8 If the parties to a written contract fail to express their agreement in the terms thereof, and they execute a new contract to express their real intent, such contract needs no other consideration.9 If a contract has been performed on one side in full, a modification of the executory part of such contract whereby the original liability of the party who is still to perform remains unmodified but an additional liability is imposed upon him, is invalid unless a new consideration supports such new promise.10 So if a written contract of sale contains a warranty a subsequent oral warranty is unenforceable unless supported by a valuable consideration.11
2 Doty v. Nixon, 109 Mich. 266; 67 N. W. 116.
3 Grist v. Williams, 111 N. C. 53; 32 Am. St. Rep. 782; 15 S. E. 889.
4 Sheffield Furnace Co. v. Coke Co., 101 Ala. 446; 14 So. 672; Spinning v. Drake, 4 Wash. 285; 30 Pae. 82; 31 Pac. 319.
1 Weatherford v. McCrocklin (Ky.), 34 S. W. 24. 2 Mt. Holly, etc., Co. v. Caraleigh, etc., Works, 72 Fed. 244; 18 C. C. A. 535; Campau v. Detroit, 106 Mich. 414; 64 N. W. 336; Rowland Lumber Co. v. Ross, 100 Va. 275; 40 S. E. 922; Skobis v. Ferge, 102 Wis. 122; 78 N. W. 426.
3 Campau v. Detroit, 106 Mich. 414; 64 N. W. 336.
4 Harvey v. Morey, 22 Colo. 412; 45 Pac. 383.
1 Pioneer Savings & Loan Co. v. Nonnemaeher (Ala.), 30 So. 79; Badders v. Davis, 88 Ala. 367; 6 So. 834; Carter v. Rhodes, 135 Cal. 46; 66 Pae. 985; Jones v. Haines, 117 Ia. 80; 90 N. W. 518; Pease v. Mc-Quillin, 180 Mass. 135; 61 N. E. 819; Thomas v. Barnes, 156 Mass. 581; 31 N. E. 683; Bowman v. Wright, 65 Neb. 661; 91 N. W.580; affirmed on rehearing. 65 Neb. 666; 92 N. W. 580; Bryant v. Thesing, 46 Neb. 244; 64 N. W. 967; Dyer v. Irrigation District, 25 Wash. 80; 64 Pae. 1009; Long v. Pierce County, 22 Wash. 330; 61 Pae. 142; Brown v. Everhard, 52 Wis. 205; 8 N. W. 725.
2 Carter v. Rhodes, 135 Cal. 46; 66 Pae. 985.
3 Pease v. McQuillin, 180 Mass. 135; 61 N. E. 819.
4 Main Street, etc., Ry. Co. v. Traction Co., 129 Cal. 301; 61 Pae. 937; Pence v. Adams. 116 la. 462; 89 N. W. 1065; Mclntyre v. Mining Co., 20 Utah 323; 60 Pae. 552.
5 Pence v. Adams, 116 la. 462; 89 N. W. 1065.