Prior permission for immovable property

I have purchased immovable property(flat) through Housing loan, during leave period and immediately on return from leave intimated my department(central Govt) about the purchase, however could not seek prior permission from my dept as i was on leave and away from my place of duty. Now my dept has asked why prior permission was not taked as per ccs rules 1964 18(3) ...etc ....plz tell me how to reply them.