Havells India — Not returning amount paid

Address: Pooja Electricals , 222, A.J.C. Bose road , Kolkata 700017

I have purchased a havellsOpus Prime fan from Pooja Electrical in Kolkata.
The very next day I went to return it and asked them to hive the other model in replace of that but it was not available with them so I asked them to refund me the amount i.e Rs 3600 so they gave us the date and said you can collect on that day . Today when we went to get the refund amount they said they will not return the full amount will deduct Rs 500 as Gst amount . I don’t get this why ?