Mich. Comp. Laws § 408.878

Sec. 8. (1) This act does not prohibit a governmental unit from awarding a contract, grant, tax abatement, or tax credit to a private owner, bidder, contractor, or subcontractor who enters into or who is party to an agreement with a labor organization, if being or becoming a party or adhering to an agreement with a labor organization is not a condition for award of the contract, grant, tax abatement, or tax credit, and if the governmental unit does not discriminate against a private owner, bidder, contractor, or subcontractor in the awarding of that contract, grant, tax abatement, or tax credit based upon the status as being or becoming, or the willingness or refusal to become, a party to an agreement with a labor organization. (2) This act does not prohibit a contractor or subcontractor from voluntarily entering into or complying with an agreement entered into with 1 or more labor organizations in regard to a contract with a governmental unit or funded in whole or in part from a grant, tax abatement, or tax credit from the governmental unit.