The Secretary of the Interior, in his discretion, may allow quarters, fuel, and light to employees of the Indian Service whose compensation is not prescribed by law, the salaries of such employees to be fixed on this basis and the cost of providing quarters, fuel, and light to be paid from any funds which are applicable and available therefor: Provided, That this authorization shall be retroactive to the extent of approving any expenditures for such purposes authorized by the Secretary of the Interior prior to June 7, 1924.