The board of supervisors of any county that has created a local district pursuant to § 15.2-4603 may advance funds, or provide matching funds, from money not otherwise specifically allocated or obligated, from whatever source received or generated, including without limitation general revenues, special fees and assessments, state allocations, and contributions from private sources, to a local district to assist the local district to undertake the project for which it was created. The Commonwealth Transportation Board may allocate funds to a district only from the construction district or districts in which such transportation district is located pursuant to the highway allocation formula to assist the district with an approved project as provided by law.