Unilateral variation of contract

If a party to the contract tries to alter that contract unilaterally are they in breach of any law?

For example, A and B have a contract that states that at a date in the future B will purchase a product for A. Party A has already met its obligation in paying for the product. Before the date B states that the amount is insufficient to provide the product and states that A must purchase the product. Is the statement itself in breach of contract law?