How do i find / search or view invoice

I have created invoice and gave it to my customer. After one week later he comes to me & wants to return one item from invoice.....I go to the setup=>View or Print Transactions to .....  I cannot find this invoice because ... in invoice I got the reference number....but there is no option to search by reference number.... I can search only system generated invoice number. System generated invoice number is not in the invoice while I print it for customer ....so it is very complicated to search invoice that I generated... please give me the solution

thank- nazmul