Dev thinks its an insult to his mom if Sona returns the check. I have been thinking about the same, and could think of one reason only. Please suggest your opinions why Dev finds it insulting.
My take: Dev thinks his mom is upset because Dev hid the money lending part from her. He thinks that she doesn't care about the money per say. And perhaps that right to some extent. So, if Sonakshi returns the money to her, its rubbing into Ishwari's face that she is so petty and she cares only for money, when she actually cared for sharing info by Dev.
Sonakshi also knows that returning money is insulting to Ishwari, but then Ishwari also called Boses frauds and using Dev for money. She wants to remove that allegation from her family's name and reputation.
Dev will say doing a new mistake to revenge a previous mistake is not right, but does he realize that even once to make Ishwari and GKB apologize for their baseless allegations to Bijoy Bose? I am sure, if Ishwari had said something, instead of maniacally mumbling, this situation would not arise. Even after the intentional slap (and YES, I will say it was intentional, even though whoever can scream it was accidental), she could have said something. But she really wants the situation to go bad.
I am now only waiting for Sonakshi to show Ishwari the mirror. After that I don't care what Dev thinks of her listening to their half conversation, or he divorces her (good for Sonakshi only), or if Ishwari gets a heart attack or they come on the road.