No. Even though a buyer or seller can propose an amendment to the contract at any time, merely proposing an amendment to a contract–or refusing to accept a proposed amendment–does not give either party a unilateral right to terminate an existing contract. The contract is only changed after the parties sign the amendment signifying their agreement. Without a fully executed amendment, the original contract remains in effect as written.
Keep in mind that if the buyer purchased a termination option that had not yet expired, the buyer could terminate the contract for any reason.