Section 2--616. Procedure on Notice Claiming Excuse

(1)  Where the buyer receives notification of a material or indefinite
  delay or an allocation justified under the preceding section he  may  by
  written  notification  to  the  seller as to any delivery concerned, and
  where the prospective deficiency substantially impairs the value of  the
  whole  contract  under the provisions of this Article relating to breach
  of installment contracts (Section 2--612), then also as to the whole,
         (a) terminate and thereby discharge any unexecuted portion of the
             contract; or
         (b) modify the contract by agreeing to take his  available  quota
             in substitution.
    (2)  If  after  receipt of such notification from the seller the buyer
  fails so to modify the contract within a reasonable time  not  exceeding
  thirty days the contract lapses with respect to any deliveries affected.
    (3)  The  provisions  of  this section may not be negated by agreement
  except in so far as the seller has assumed a  greater  obligation  under
  the preceding section.