Performance of a legal duty owed to a promisor which is neither doubtful nor the subject of honest dispute is not consideration; but a similar performance is consideration if it differs from what was required by the duty in a way which reflects more than a pretense of bargain.
one of the alternative performances would have been consideration and there is or appears to the parties to be a substantial possibility that before the promisor exercises his choice events may eliminate the alternatives which would not have been consideration.
A promise modifying a duty under a contract not fully performed on either side is binding
if the modification is fair and equitable in view of circumstances not anticipated by the parties when the contract was made; or
to the extent provided by statute; or
to the extent that justice requires enforcement in view of material change of position in reliance on the promise.
Copyright, The American Law Institute
Restatement Second of Contracts § 89(c) Explanation
R2C § 89 basically covers if it becomes a really bad deal for one side so they agree to change the terms without consideration to get it done. Since it only got completed because they agreed to change it, it wouldn't be fair to make the person go back to the original. It is obviously extremely rare.
The cases about it basically fall into the pattern of:
Person A agrees to build a house for $100,000.
The price of building materials shoots up.
Person A says, "I know I agreed to build this for $100,000, but the materials alone are $150,000 now. I'm not going to be able to finish. I'll just pay you for the breach of contract."
Person B says, "I really want this house. I'm fine with a price increase. Please, just agree to keep building."
Person A finishes building the house and sends a bill for $200,000.
Person B says, "Ha! You didn't offer any consideration. You only agreed to do something you already agreed to do! Therefore, I'm only paying you $100,000, and you're out for $100,000."
§89(c) says that's not gonna fly. Since Person A would have lost less money if he just broke the contract than if he kept building on Person B's promise, the modification will stand.
By contracting around, e.g. tearing up the old contract and writing it again.
Unless excluded or modified ( [UCC § 2-316]), a warranty that the goods shall be merchantable is implied in a contract for their sale if the seller is a merchant with respect to goods of that kind. Under this section the serving for value of food or drink to be consumed either on the premises or elsewhere is a sale.
Where the seller at the time of contracting has reason to know any particular purpose for which the goods are required and that the buyer is relying on the seller's skill or judgment to select or furnish suitable goods, there is unless excluded or modified under the next section an implied warranty that the goods shall be fit for such purpose.
A promise which the promisor should reasonably expect to induce action or forbearance on the part of the promisee or a third person and which does induce such action or forbearance is binding if injustice can be avoided only by enforcement of the promise. The remedy granted for breach may be limited as justice requires.
A charitable subscription or a marriage settlement is binding under Subsection (1) without proof that the promise induced action or forbearance.
A promise made in recognition of a benefit previously received by the promisor from the promisee is binding to the extent necessary to prevent injustice.
Restitution is an obligation imposed by law on ground of justice and equity. Its purpose is to prevent unjust enrichment. Unlike express contracts or contracts implied in fact, restitution does not rest upon the assent of the contracting parties.
The "officious intermeddler doctrine" holds that where a person performs labor for another without the latter's request or implied consent, however beneficial such labor may be, he cannot recover therefor.
An exception is that of emergency aid, where the service is needed to prevent the others' bodily harm and the helper is a doctor and therefor doesn't give the implication of gratuitousness.
The fact that a recipient has obtained a benefit without paying for it does not of itself establish that the recipient has been unjustly enriched.
A valid contract defines the obligations of the parties as to matters within its scope, displacing to that extent any inquiry into unjust enrichment.
There is no liability in restitution for an unrequested benefit voluntarily conferred, unless the circumstances of the transaction justify the claimant's intervention in the absence of contract.
Liability in restitution may not subject an innocent recipient to a forced exchange: in other words, an obligation to pay for a benefit that the recipient should have been free to refuse.