If any funds are paid to any county from the motor vehicle fund in excess of the amount estimated by the department of transportation and the excess funds have not been included by the county legislative authority in the then current county road budget or if funds become available from other sources upon a matching basis or otherwise and it is impracticable to adhere to the provisions of the county road budget, the legislative authority may by unanimous consent, consider and adopt a preliminary supplemental budget covering the excess funds for the remainder of the current fiscal year.