Hi,
Only in the backend invoice ?
I have update the HikaShop package in order to display the order_number if the invoice number is not generated.
Please re-download HikaShop 2.0 and it should fix your problem.
If there are other pages where the order_number should be displayed when the invoice_number does not exists, please reply in this thread.
Best regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.