Sale Invoice Raised but Goods not Sent

Raju Roy (Accountant ) (80 Points)

10 May 2021  
Respected Sir / Madam,

We have raised Sale Invoice for sending goods to a customer against Advance received, in September 2020, but unfortunately material not sent and Advance money was refunded to customer and order was canceled accordingly.
At present, how can I adjust or reverse the liability because GSTR 1 also filed in October 2020. As on March 2021, Debtor's Balance is reflected in our books.
Kindly advise.