There is hereby established in the Treasury a “Working capital fund”, to be available without fiscal year limitation for expenses and equipment necessary for the maintenance and operation of such administrative services as the Administrator determines may be performed more advantageously as central services: Provided, That any inventories, equipment, and other assets pertaining to the services to be provided by such fund, either on hand or on order, less the related liabilities or unpaid obligations, and any appropriations made hereafter for the purpose of providing capital, shall be used to capitalize such fund: Provided further, That such fund shall be paid in advance or reimbursed from funds available to the Agency and other Federal agencies for which such centralized services are performed, at rates which will return in full all expenses of operation, including accrued leave, depreciation of fund plant and equipment, amortization of automated data