Export sales not realised payment not received from foreign client

Dear Sir,

export sales is not realised (payment not received from client)

goods were exported on payment of duty. rbi extension availed

but now we have realised we cannot recover payment from client.

how to write off . (IGST PAYMENT ALONG WITH INTEREST) TO BE PAID BACK

DUTY DRAWBACK TO BE PAID BACK. WRITE OFF PROCEDURE FROM GST POINT OF VIEW