Your running up against the browsers "Save File As" dialog box. No, you can't from that dialog specify the download URL.
You have to have the user select the URL before you issue the content type header. Then its up to your code to retrieve the data from that URL or redirect the user to that URL. How you want that to work is your decision.
I'm new to this board (cool community by the way), but demanding to have code given to you so you could use it in a job that is paying you to write code is rather insulting. Last time I checked, the folks here that are posting help are volunteering to do it (aka: not getting paid). Life is rough, but do some research and learn so you earn that buck or two... [/end rant]