Procedure to surrender Drawback against unrealised bills

Write a letter to the asst/addl commissioner stating the shippinh bill number and reasons for failure to realise the amount and enclose a DD for the drawback amount mentioned in the shipping bill.

For partial realisation you can calculate pro rata. Talking about interest, 18℅ p.a is the rate. However, if you refund suo moto there is a chance they waive the interest amount. Hence just refund the drawback amount and visit the Customs office.