Question Summary:
Is it permissible to make a secret Nikah?

Question Detail:

A boy has proposed to get married to a girl and she has accepted. Both parties’ families are happy. However as the boy is still not financially stable to support the girl as yet but will be in a few months’ time. The families want the nikaah to take place then.
The boy and girl feel as they are talking they rather make the nikaah immediately so as not to commit sins out of nikaah. Can they have a secret nikaah where only they know about it? Can they have the public nikaah again to please their families?

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
 
The purpose of Nikah is to avoid committing sins and to preserve ones chastity. If the boy and girl cannot control themselves and are certain they will engage in illicit and out of Nikah relationship, then it is wajib for them to marry immediately.
It is advisable to make the family understand and make them part of the Nikah. While a secret Nikah will be valid, one will have to face the negative consequences of concealing the Nikah from near and dear ones once they know of the Nikah. Furthermore, the possibility of pregnancy after the secret Nikah and time frame of birth before the official open Nikah may also prove to be challenging.
And Allah Knows Best. 
Mufti Ebrahim Desai