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With Nginx you can log the amount of bandwidth used pushed to a log file by using the log_module but this is not exactly what you want, but can help achieve what you wish.
So, now you will have logs that you can parse to get the file sizes and total bandwidth used and then have a script that updates a database and then you can then authorize future downloads if their limit is reached or within some soft limit range.
Server Fault with similar question
Another option is, making an assumption, keep the file sizes in a database and just keep a tally at the request so when ever they hit a download link it immediately increments their download count and if it is over their limit then just invalidate the link else make the link valid and pass them over to Nginx.
Another option would be to write a custom Nginx module that performs the increment at a much more fine grained level, but this could be more wok than your situation requires.