There is established a working capital fund which shall be available without fiscal year limitation for expenses necessary, including the purchase or construction of buildings and improvements within the limitations thereon set forth in the appropriations for the Forest Service, for furnishing supply and equipment services in support of programs of the Forest Service. The Secretary of Agriculture is authorized to transfer to the fund, without reimbursement, and to capitalize in the fund at fair and reasonable values, such receivables, inventories, equipment, and other assets as he may determine, and assume the liabilities in connection with such assets: Provided, That the fund shall be credited with advance payments in connection with firm orders and reimbursements from appropriations and funds of the Forest Service, other departmental and Federal agencies, and from other sources, as authorized by law, at rates approximately equal to the cost of furnishing the facilities and service.