Mich. Comp. Laws § 141.126
Any money remaining in the accounts of the public improvement at the end of any operating year, which under the provisions of section 22 shall be considered surplus, may be transferred to other accounts of the public improvement or may be used for the purpose or purposes as the governing body may determine to be for the best interests of the borrower, unless some other disposition shall have been made in the ordinance authorizing the issuance of bonds under this act. In the event that money of the public improvement is insufficient to provide for the current expenses of the operation and maintenance account or the bond and interest redemption account, any money or securities in other accounts of the public improvement shall be transferred first to the operation and maintenance account and second to the bond and interest redemption account to the extent of any deficits in those accounts.
MCL 141.126