Mich. Comp. Laws § 247.432
Donations of money may be received by either the state highway commissioner or the county road commissioners for the benefit of any road sought to be improved in accordance with the provisions of this act. Such money shall be paid to the treasurer of the county in which such special assessment district, or the greater portion thereof, is situated. Donations received before the preparation of the assessment roll shall be so applied as to decrease proportionately special assessments apportioned against individual property owners. In the event that the proposed improvement is not made, money so donated shall be returned to the donor on the warrant of the state highway commissioner or the board of county road commissioners as the case may be.
MCL 247.432