Seems to me like Bijoy must have asked for the pre-nup agreement. He still doesn't trust the Dixit's and so perhaps want to see their true colors.
Also he wants to make sure that the Dixits will think ten times before throwing Sona out of their life if they sign the agreement.
As such according to law Sona might anyways get 50% of the property .. but Bijoy cannot trust the dirty and manipulative games they might play in case the divorce situation occurs.
I think Dev might not be happy when his lawyer informs about it and throws the agreement .. Dev perhaps wants the marriage to be based on mutual trust. But after seeing Dev's past actions, Bijoy cant trust him.
I guess when Ishwari comes to know about it, she might ask Dev to sign the agreement.
As per this news segment, Bijoy is happy after Ishwari agrees for agreement :
https://www.youtube.com/watch?v=IvPxnZrTSnk
Hmmm !! Seems like the CV's are going to bring in divorce track in future segments :-(