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 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.1 If a contract has been fully performed by one party, his promise not to enforce the executory covenants of such contract against the adversary party, is unenforceable unless there is some additional consideration.2 If a contract for the sale of goods has been performed by delivering the goods, a subsequent agreement discharging the liability of the adversary party is invalid unless supported by some additional consideration.3 If a written contract of sale contains a warranty, a subsequent oral warranty is unenforceable unless supported by a valuable consideration.4
9 See Sec. 514 et seq.
10 See Sec. 589.
11 See Sec. 590.
There are, however, a number of exceptions to this general principle which in themselves are really exceptions to the general theory of consideration. As they have been discussed elsewhere in detail, a mere reference in this connection is all that is necessary. If the contract in question is a negotiable instrument, the cancellation of such instrument by the holder thereof, with intent to discharge the maker from liability, has this effect, and no consideration therefor is necessary.5 In some jurisdictions, statutes have been enacted which give to the ordinary unsealed release or to the receipt in full, substantially the same effect as that of the common-law release under seal; and by such statutes such a release or receipt is made to operate as a discharge without consideration.6 In some jurisdictions the courts have attempted to reach substantially the same result without the aid of legislation by treating a promise by the creditor to discharge the debtor as a gift of the debt by the creditor to the debtor; and, accordingly, by holding that such promise operates as a discharge even without consideration, since it can be treated as a gift.7