[Koha] additional text on manual invoice(fines)

Regards.. I am a newbie to Koha. Would like to know how am going to print a unique auto genertated number (receipt number) on the invoice or any unique number which is not from the database. tq ROZITA BT HASSAN Bahagian Teknologi Maklumat & Komunikasi Perbadanan Perpustakaan Awam Negeri Perak (T) 05-2491883 / 016-5355848 (F) - 05-2491887