1. Where the buyer receives notification of a material or indefinite delay or an allocation justified under the preceding section [§ 8.2-615] 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 title relating to breach of installment contracts (§ 8.2-612), 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 [§ 8.2-615].