Automatic Link change

On our system here: [login to view URL] (username: onlinesample[at][login to view URL] password: 123), when users click on the link of a free application (with no Purchase url in the database), it installs the application. However, when they click on a commercial application (with a Purchase url), still tries to install the application. What I want is that the system will check if there is a purchase url, and if so, it sends the user to that link, otherwise it just continues as normal. Lower budget.