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.
The alleged consideration in a contract between A and B may be the act of B in performing the contract which B has already entered into, or in promising to perform such contract; and the act which B performs or promises to perform, may be one which he is bound to perform by a contract into which he has already entered with A, or it may be an act which he is required to perform by a contract into which he has already entered with a third party, such as X. If the act which B performs or promises to perform, is one which he is bound to do under a contract which he has already entered into with A, and such act is something other than the payment of money, the courts are at variance as to the sufficiency of such act or promise on the part of B as a consideration for A's promise; but the great weight of authority is that such act or promise is not a consideration, since B is doing only what he is already bound to do by his contract and since he is performing it for A, to whom he is already bound by such contract. B gives nothing except what he was already bound to furnish, and A receives nothing to which he was not already entitled; and, accordingly, such performance is not sufficient consideration for the second contract.1 Accordingly, it is very generally held that rendering services under a prior contract is no consideration.2 If A has made a contract with B, through B's agent, X, to care for B, A's making such contract again with B in person, is no consideration for B's promise to make a bequest to A.3 If, without legal justification, one party to a contract breaks it, or threatens to break it, and to induce performance on his part the adversary party promises to pay more than was provided for by the original contract, there is on principle no consideration for such promise, as the party who threatens to break the contract does, when he finally performs it, no more than he was bound in law to do.4 Accordingly, the weight of authority is that there is no consideration for such promises.5 A promise to pay an additional consideration to a vendor of realty to induce him to perform his contract to convey,6 such as a promise to pay an additional amount to one of two joint vendors under a contract for the sale of realty to induce him to execute a deed in performance of such contract;7 a promise by the vendee of realty to pay rent in addition to the payments required by the contract;8 a promise by a debtor to pay his debt, as consideration for a promise by the mortgagee to release certain realty from the lien of such mortgage;9 and a promise by a mortgagee to pay other debts of the mortgagor to induce such mortgagor to execute the mortgage in accordance with the terms of the original contract,10 are none of them supported by consideration. The return of a check which was not given for a legal obligation, and the payment of which the maker could rightfully stop, was held to be no consideration for the maker's promise.11 Promises by a surety to induce the principal to pay the note;12 a promise to induce a debtor to pay the debt;13 a promise to induce a debtor to give his note for a debt;14 a promise by an accommodation endorser made in consideration of the payment of such note by the party for whose benefit it was endorsed;15 a promise to induce an architect to complete a building contract;16 a promise of additional compensation to induce a workman to perform his contract;17 additional cornpensation to a sub-contractor for work required by the contract;18 or promise to do additional work, made after the contract was completed, but before notes were signed, as provided by the terms of the contract,19 as a promise by a builder, after completing a house according to contract, to put on an additional coat of varnish;20 or a promise to pay to a sailor a higher rate of wages than was originally agreed upon,21 are not supported by consideration. If A and B had entered into a contract by which B was to plant and gather a crop on A's land for a certain share of the crop and for a certain additional compensation, A's subsequent promise to guarantee a certain minimum profit to B, in consideration of B's performance of such contract, is not supported by sufficient consideration.22 So a promise by a mortgagor to assign an insurance policy after loss, which policy he had agreed to assign when the loan was made;23 a reinstatement of a forfeited policy in accordance with the terms of the original contract;24 a direction in a will for repayment of a loan made to testator by his wife;25 or •executing a deed in accordance with a contract so to do,26 are none of them considerations for promises based thereon. If A has agreed to convey land to B for a certain price, and at the time for performance B refuses to perform unless A also agrees to procure for B the title of certain remaindermen in a distinct tract of land, such latter promise by A is without consideration.27 If A employs B to act as detective, B can not recover a reward which A has offered for the arrest and conviction of criminals, whom it was B's duty to arrest and convict.28 If A and B, who are directors of a corporation, have agreed to pay the debts of such corporation in a certain proportion, a subsequent promise by which A agrees to pay a greater proportion of such indebtedness, in order to induce B to pay a portion thereof, is without consideration.29 No legal right has been given up by the promisee and none has been acquired by the promisor. Unless a new theory of consideration is to be evolved, the foregoing cases must be regarded as correct. A prior oral agreement is not a consideration for a subsequent promise which is made to induce a party to such oral agreement, who has subsequently entered into a written contract differing from such oral agreement in certain respects, to perform such written agreement, even if the subsequent promise conforms substantially to the terms of the original oral contract.30
2Dallavo v. Dallavo, 180 Mich. 350, 155 N. W. 538.