PreExisting Duty Rule (Modifications under Common Law)
A promise to do something you are already legally obligated to do (generally, or based on a prior contract) is NOT CONSIDERATION
Exceptions to Preexisting Duty Rule
Is promising PARTIAL PAYMENT in exchange for a release from a DEBT binding?
NOT if the debt is currently due and the amount is undisputed. this would be a preexisting duty!
Otherwise (if debt not due yet) sure. This is not yet a preexisting duty.
Rule for Modifications under UCC
Modification has to be in good faith
If so, binding even without consideration.
Rule for Modifications under Common Law
PreExisting Duty Rule