Every organisation is having Unions/Associations and they pay subscripttions monthly. Contact a colleague or visit to the office of Association or Union and acquint with the laid down procedure as per employment terms and conditions.
If the procedure is cumbersome and not convenient to you, you can seek such opinion from Advocate and keep the flat in any of your family members.
Sanjit ji generally prior approval is sought when you want house building advance hba or from the funds by your deptt.
In another case an Appl for intimation in advance be submitted enclosing proforma 18(2)/18(3)
The general principle is that prior permission is required only when the purchase is from a person having official dealings with you; else, subsequent intimation within the time specified would be sufficient.