R2d § 89
Modification of Executory Contract
A promise modifying a duty under a contract not fully performed on either side is binding (a) if the modification is fair and equitable in view of circumstances not anticipated by the parties when the contract was made; or (b) to the extent provided by statute; or (c) to the extent that justice requires enforcement in view of material change of position in reliance on the promise.
Professor's notes
Elements: a promise modifying a duty under a contract not fully performed on either side is binding (a) if the modification is fair and equitable in view of circumstances not anticipated by the parties when the contract was made; or (b) to the extent provided by statute; or (c) to the extent that justice requires enforcement in view of material change of position in reliance on the promise.
Angel v. Murray operationalizes (a): garbage hauler's request for more money after an unanticipated population boom was a fair-and-equitable modification.
Common misunderstanding: students think § 89 abolished the pre-existing duty rule. It did not. § 89 carves a narrow safety valve for unanticipated circumstances. Coerced "hold-up" modifications (Alaska Packers) still fail. The rule is lifnim mishurat hadin: moving beyond strict din to recognize fairness in changed circumstances.
Cases that operationalize this rule
Text
R2d § 89. Modification of Executory Contract.
A promise modifying a duty under a contract not fully performed on either side is binding
(a) if the modification is fair and equitable in view of circumstances not anticipated by the parties when the contract was made; or
(b) to the extent provided by statute; or
(c) to the extent that justice requires enforcement in view of material change of position in reliance on the promise.