Banner Exchange / Top sites

Wanting an adserver that allows advertisers to gain credit by sending links in to my site. For example, if a user signs up, adds a banner and URL and then gets a unique link to my site ([login to view URL]) and then promotes that link. We then count how many hits are referred by each user and then we set that percentage of adverts to show that users banner on my site. We also need stats to show each user how many links they sent in, what percentage of the ads they will get and number of displays / clicks etc. Stats should renew weekly - send 30% of the traffic in for that week and you get 30% of the banners shown. If there is a suitable open source script that can be adapted then that is fine.