Gst export invoice and conversion rate

EXPORT INVOICE CREATED ON 31/07/2018 - 1000USD- CONVERSION RATE 67.IGST ALSO APPLIED ON 67000/- (EWAY BILL ALSO GENERATED). SHIPPING BILL DATED 04/08/2018- CONVERSION RATE USED - 67.80 WHICH CHANGES THE FOB AND INVOICE VALUE IN SHIPPING BILL. MY QUESTION IS - WHAT WILL BE THE TREATMENT OF SUCH TRANSACTION AS IT WILL DIFFER MY INVOICE VALUE IN BILL AND INVOICE VALUE OF SHIPPING BILL.AND WHAT AMOUNT OF IGST HAVE TO PAY AS PER MY INOVICE OR SHIPPING BILL?? AND WHAT WILL BE IMPACT ON MY EWAY BILL?