This is very much a possibility, it would just require that you make an update to your db when you get confirmation of the transaction. Then you would have a recordset on your download page that would select from this table where the AllowDownload was set to the correct value. The query you would need would be a little more complex than that though, you would need to join this table with your items table to get the details about the file.
Once crafted this way you would use the bindings from this recordset in the download server behavior, you could trigger the server behavior on the recordset not being empty.
