Air India — Partial refund given even when the flight was cancelled by Air India

I booked Air India tickets SINGAPORE TO UDAIPUR (Singapore to Delhi & Delhi to Udaipur) on 4th May'2022. Air India cancelled one of my flights (Delhi to Udaipur) due to which I had to cancel the entire ticket. When I spoke to one of the customer service representatives, the lady told me she can cancel the ticket and I will get my FULL refund (I have the voice recording). She then cancelled my ticket (27th May) and I was asked to fill up the feedback form .I received only a partial refund (SGD 779) instead of full refund(SGD 1179) with no explanation why $400 had been deducted as the flight was cancelled by Air India itself. I have been chasing them for almost 6 months and yet there has been no update. Every time I speak to their representative, they write a note to the refund department and still no action has been taken. Can you plz help me to get my remaining $400 back?