Mich. Comp. Laws § 450.1305

Sec. 305. (1) A subscription for shares made before or after organization of a corporation is not enforceable unless in writing and signed by the subscriber. (2) A subscription for shares of a corporation to be organized is irrevocable and may be accepted by the corporation for a period of 6 months, unless otherwise provided by the subscription agreement or unless all the subscribers consent to its revocation. (3) A contract with a corporation to purchase its shares to be issued is a subscription agreement and not an executory contract to purchase shares, unless otherwise provided in the contract.