Noted that shipment was via eBay's GSP programme.
Noted that buyer opened a return case citing reason "Item is damaged".
I would have expected you to have received a message from eBay advising you that a return case is opened and that your funds are on hold and as this is a GSP shipment eBay at this stage do not require any further information from you and they will contact you in a few days.
If case is opened then if allowed then message your buyer through the case if not then through eBay's messages asking him to obtain a quote from his local postal service provider to return the item back to you.
I do note your buyer's comment about non delivery but you asking him for return postage cost on a SNAD case would hep you as trying to resolve your buyer's complaint and I would expect your buyer to respond stating that he has nothing to return as he has not received his item and this admission should result in eBay closing the case without a refund to buyer and releasing your held funds as an INR case would have failed as track& trace shows item delivered.
if case was opened in the last 1 or 2 days then on the case itself eBay would show a date that eBay can be asked to step in and not earlier. This allows seller and buyer time to reach an agreement/ resolve the case.
I would call eBay's CS and discuss this case with them and ask them to read all messages and they will put some notes on the case for eBay's resolution team.
In the unlikely event that you did not receive any such notification/ message from eBay and that your funds related to this transaction are not on hold then I am confused by your post and again I would recommend that you contact eBay's customer services and ask them to view the order and messages and whether there is a return cases for this order shipped through GSP.
Good luck
