Gstr 3b

hello.
we are exporters and made the shipment with the payment of IGST in the month of May 2018..the returns were filed in June..however the refund of Igst was not received.. when checked with our returns we found that in Gstr 3b the export value had to be entered in zero rated column has been wrongly entered under nil rates thereby the tax liability was not set off or paid..
I know there is no scope for revising the returns
let me know if we can pay the taxes while filing for month of July and get the refund later

thanks
Kiran