Can I delete an app I got on sale?
From play store support:


  You can use apps you bought on Google Play on any Android device
  without paying again. However, each device must have the same Google
  Account on it.
  
  You can:
  
  
  Reinstall an app you bought but deleted
  Install an app on more than one Android device. 
  Install an app on a new Android device. 
  


Even though the app you downloaded was free now, being that usually it is a paid app, Google will count it as if you paid $0.00 and therefore it's counted as if you bought it. And as quoted above, purchased apps can be reinstalled later.