I might have totally the wrong end of the stick here, but I thought this sort of task would be better suited to the Fee based roles. Not sure of the ins and outs, but if you specify a 'payment processor' in Admin -> Site Settings -> Advanced Settings -> Payment Settings, and then create a Role that will have permission to do the download, and in the Edit Security Role page for that role, under the Advanced Settings section you can set a one-time fee for that role.

I've no idea how you get the user to pay the fee and take on the Role - but I always assumed that this functionality was for the purpose you outlined. Would be interested to know if you have any luck with it.