Mich. Comp. Laws § 46.188

Sec. 18. Actions heretofore taken by the county or any unit of government and all bonds heretofore issued under this act as originally adopted or subsequently amended are hereby validated. A county acting under this act as originally adopted or subsequently amended, or any unit of government, shall not contest the validity of any such bonds or any contract which provides the security therefor or any action taken by the county or unit of government after the bonds have been sold and delivered and the county has received the consideration therefor.