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.
After the contract is made, new terms can not be added by either party without the consent of the other.1 If after acceptance the acceptor insists on a modification of the original contract in which the offeror does not acquiesce, such insistence can not avoid the contract. Hence, the acceptor can subsequently enforce the original contract in the absence of facts to create an estoppel.2 If the adversary party accepts such modification, a new contract is formed.3 If he rejects such modification the original contract remains in force. If he refuses performance unless such modification is agreed to, there is a case of breach by renunciation.4