Mich. Comp. Laws § 46.174

Sec. 4. When the county board of commissioners of a county has authorized and directed the establishment of any of the improvements, facilities, or services authorized by this act, the county agency shall establish just, equitable, and uniform rates, charges, or assessments to be paid to the county for the services rendered thereby. The complete and actual cost of improvements and financing thereof may be included in the amounts fixed for rates, charges, or assessments for services rendered by the county. Where the improvements or facilities are to be acquired, constructed, and financed pursuant to the provisions of sections 5a, 5b, and 5c, the rates, charges, or assessments for services rendered by the improvements or facilities shall be set and thereafter changed in the amount and manner provided by contract between the county and the unit or units of government.