Mich. Comp. Laws § 423.215a
An employee of a public fire department may volunteer for or seek and accept part-time or paid on-call employment with another fire department if that employment does not conflict with his or her performance of the original employment as determined by the original employer. This section does not create a right for a full-time employee of a public fire department to accept full-time employment with another fire department. A local unit of government shall not adopt or apply an ordinance, rule, or policy in conflict with the right granted an employee under this section. Collective bargaining between a public employer and a bargaining representative of its employees shall not include the subject of a prohibition on an employee volunteering for or obtaining paid on-call employment with another fire department.
MCL 423.215a