Discharge or modification of a duty to an intended beneficiary by conduct of the promisee or by a subsequent agreement between promisor and promisee is ineffective if a term of the promise creating the duty so provides.
In the absence of such a term, the promisor and promisee retain power to discharge or modify the duty by subsequent agreement.
Such a power terminates when the beneficiary, before he receives notification of the discharge or modification, materially changes his position in justifiable reliance on the promise or brings suit on it or manifests assent to it at the request of the promisor or promisee.
If the promisee receives consideration for an attempted discharge or modification of the promisor's duty which is ineffective against the beneficiary, the beneficiary can assert a right to the consideration so received. The promisor's duty is discharged to the extent of the amount received by the beneficiary.