The authority of the county engineer shall be limited to the expenditure of funds appropriated by the county commission. The county commission shall fix and determine the amount of funds which will be available for the purpose of inspecting, building, maintaining, and constructing public roads, bridges, and ferries of Covington County for the ensuing fiscal year, beginning on October 1. That amount shall not be exceeded, except that the county commission, from time to time, within any period, may amend the amount allowed to be expended by the county engineer during the period, provided that the authorization does not conflict with other provisions of law.