Server side validation for purchases

Hi Everyone,

I’m building an app which will be integrated with RevenueCat. The idea is that user can purchase coins (internal game currency which uses can use only inside application).

I want to implement server side validation to make sure that user has purchased the coins so i can add the coins to their game account.

The user will make a purchase in mobile app and then I want sent the transaction to server for validation and adding the coins. 

How I can do this on java backend?

Thanks.