Mich. Comp. Laws § 124.608
An authority may enter into a contract with any incorporating municipality for the provision of emergency services in the incorporating municipality for a period not exceeding 30 years. The emergency services may be established or funded in conjunction with any municipal emergency services, and any municipal emergency service may be delegated by contract to an authority. The charges specified in a contract shall be subject to increase by the authority, if necessary, in order to provide funds to meet its obligations. An authority may also enter into contracts with a city, village, or township that is not an incorporating municipality, for a period not exceeding 30 years except that the charges for services under a contract with a nonincorporating municipality may be greater than the charges to an incorporating municipality, and shall be subject to change from time to time without notice. However, existing contracts between the county sheriff's department and the municipalities shall remain in effect for the lifetime of those contracts.
MCL 124.608