Mich. Comp. Laws § 17.52: Payment for goods and services by principal executive department; time.

Sec. 2.

Unless otherwise agreed in writing, a principal executive department shall take all steps necessary to assure that payment for goods or services is mailed to a private enterprise within 45 days after the principal executive department receives the goods or services, a complete invoice for the goods or services, or a complete contract for goods or services, whichever is later.