You can now generate and distribute promo codes to current and new users on Google Play to drive engagement. Under the Promotions tab in the Developer Console, you can set up promo codes for your apps, games, and in-app products to distribute in your own marketing campaigns (up to 500 codes per app, per quarter). Consider using promo codes to reward loyal users and attract new customers.
Reference: 1) http://android-developers.blogspot.in/2016/01/create-promo-codes-for-your-apps-and-in.html
2) https://support.google.com/googleplay/android-developer/answer/6321495