Refund of money in export sales

Hi,

    We have done an export sale to the customer in United states, we have received the excess money than the sales value quoted in the invoice, which is a error done by the customer, credit excess cash in the bank.  The Banker is Asking the original Firc Copy . The same is been missed by ourselves.

On for refunding the cash to the customer , we have missed the orginial FIRC- document, . Is there any further process to refund the amount to customer.

Please help urgent