Agreed that it is way beyond the scope of UBB or any BBS software I know of to handle that sort of request.
It can be accomplished I'm sure with PHP and MySQL but it would be more purpose built than just a BBS.
You would probably allow multiple downloads from the same user and same IP but would want or need to check for different IP connections and track all of it. As for a guarantee that a file was successfully downloaded you would need something installed on the client to verify that. Having such control via no installed software and just access through a browser is probably not achievable.
.
