Restatement Second of Contracts § 270
Where only part of an obligor's performance is impracticable, his duty to render the remaining part is unaffected if
- it is still practicable for him to render performance that is substantial, taking account of any reasonable substitute performance that he is under a duty to render; or
- the obligee, within a reasonable time, agrees to render any remaining performance in full and to allow the obligor to retain any performance that has already been rendered.
Copyright, The American Law Institute