The Secretary of the Interior is authorized to make a reasonable allowance for office rent for each consolidated land office; and when satisfied of the necessity therefor, to approve the employment of one or more clerks, at a reasonable per diem compensation, for such time as such clerical force is absolutely required to keep up the current public business, which clerical force shall be paid out of the surplus fees authorized to be charged by section 84 ¹ of this title, if any, and if no surplus exists, then out of the appropriation for incidental expenses of district land offices; but no clerk shall be so paid unless his employment has been first sanctioned by the Secretary of the Interior.