Promo code not working in Android App

  1. Summary of my problem
    I am not a developer , I am client i had company work for me to develop android App , I tried to give promo codes to my users after generating it from Android play console.
    The developer of that company saying that promo code are not handled in the app it is handled by google payment gateway. I want to know if that is true because i was able to generate promo code without any issue but my question is how do we give discount for the promo code ?

  2. What have I tried so far - i tried google its not helpful so far.

  3. Please give me some light what i should or is that true that promo code are handled only by google payment gateway ?