prepaid accounting

Hi, I need to find a solution for my one-time event based accounting problem. I have users which buy prepaid cards and use them to add credit to their web accounts. User download videos later using their credits. I'm asked to use radius for authentication and accounting. I couldn't figure out how I can do this with freeradius. Is it doable with freeradius? If not, please show me the way to do this. Regards, Engin Deveci