From time to time there is authorized to be transferred from each or any appropriation or fund available for the purchase of supplies for the Indian Service, to a fund to be set up and carried on the books of the Treasury as an Indian Service supply fund, such amounts as the Secretary of the Interior may estimate to be required to pay for supplies purchased through Indian warehouses for the Indian field service; and the expenditure of the said Indian Service supply fund for the purpose stated is hereby authorized, necessary adjustments to be made thereafter to the end that each appropriation and fund ultimately will be charged only with the cost of the supplies legally chargeable thereto.