Exchange rate on export of goods

We are exporters of Spices from Tuticorin Port. We do exports under LUT When goods are removed from my go down in Trivandrum  to be transported to Tuticorin we generate e invoice and e way bill  with the prevailing customs rate of exchange. Once the goods reach Tuticorin the samples are drawn by Spices Board and tested for quality. After spices board gives the permission to export goods, they  After are taken to port for export  but at the time of let export the rate of exchange sometimes Changes. At the time of claiming refund /filing GSTR 1  there is a difference in the e invoice value and the value as shown in Shipping Bill. How should we report this is GSTR-1. As there is no provision to amend e invoice we are helpless. Kindly give your suggestions on how to resolve this issue Sajjad Sait