want to sell prepaid codes via pos devices

hi all, i want programmer to sell prepaid codes (mobile codes , internet codes ... etc) with POS devices. so we will store prepaid codes in the database and the merchant which have the POS device can print the codes with his balance ... also system administrator can add codes to the database. this system must be secure. you can bulid it from zero or use a ready system no problem :D the device is in C language. thanx