Any balance of proceeds of the sale of said bonds authorized for any project described in subsection (b) of section 10a-25b in excess of the cost of such project may be used to complete any other project described in said section if the State Bond Commission shall so determine and direct. Any balance of proceeds of the sale of said bonds in excess of the costs of all the projects described in subsection (b) of section 10a-25b shall be deemed appropriated and shall be used for the purposes of retiring such portion of the indebtedness on said bonds as may be retired by the amount of such balance.