Self write off calculation for unrealised export proceeds

Hello Sir

During the FY 2015-16 total Export was around 1.5cr, unrealised export proceed during FY 2015-2016 was 15 lacs

FY 2017-18 total export Rs.1.7cr and unrealised export proceed Rs.16000/-

till date that amount is unrealised... bank asking for Self written off certificate.
Can exporter claim self write off of Rs.15 lacs in FY 2018-2019 and also Rs.16000/-?

For this calculation of 5% for self write off , as in this case which year export proceeds to be considered for calculating this 5% (FY 2015-2016 the year of export, or the FY 2017-2018 previous year or FY 2018-2019 in the year when the amount is written off)