Mich. Comp. Laws § 450.1305: Subscription for shares.

Sec. 305.

A subscription for shares made before or after organization of a corporation is not enforceable unless in writing and signed by the subscriber.

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.

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.