40 U.S.C. § 526

1. (a)The Administrator of General Services may temporarily assign or reassign space in excess real property to a federal agency, for use as office or storage space or for a related purpose, if the Administrator determines that assignment or reassignment is more advantageous than permanent transfer. The Administrator shall determine the duration of the assignment or reassignment.
2. (b)If there is no appropriation available to the Administrator for the expense of maintaining the space, the Administrator may obtain appropriate reimbursement from the federal agency.