The Secretary of the Interior is authorized to sell and convey at public sale, to the highest bidder, under such regulations and under such terms and conditions as he may prescribe, at not less than the appraised value thereof, any abandoned day or boarding school plant, or any abandoned agency buildings, situated on lands belonging to any Indian tribe and not longer needed for Indian or administrative purposes, and to sell therewith not to exceed one hundred and sixty acres of land on which such plant or buildings may stand. Title to all lands disposed of under the provisions of this section shall pass to the purchaser by deed or by patent in fee, with such reservations or conditions as the said Secretary may deem just and proper, no purchaser to acquire more than one hundred and sixty acres in any one tract: Provided, That the proceeds of all such sales shall be deposited in the Treasury of the United States to the credit of the Indians to whom said lands belong, to be disposed of in accordance with existing law.