Any building or improvement which is provided by the department pursuant to this chapter may contain space in excess of the requirements of the department. Until needed, such excess space may be leased or let by the department at such rental and upon such terms and conditions as are approved by the Department of General Services. The rentals charged shall be sufficient to provide a reasonable return to the Department of Agriculture Fund or other fund that is subject to the jurisdiction of the director for any contributions which are made for the cost of the construction of the building or improvement until the entire amount contributed, together with reasonable interest, has been returned.