You should not need to pay for the Slingplayer app again. The records of your purchase should have been saved in the account you use for downloading and purchasing apps in the app store your phone uses. What you need to do is log in to whichever app store you use, using the same information as you did when you first purchased the app. It should then let you download it for free.
If you log in to your phone's app store using the correct information, and it still does not let you download the app for free, contact the appropriate app store (Google for Android phones, tablets, Apple for iPhones, iPads, etc.). They should be able to sort it out for you.
Hope this helps!