For non root users try using LUCKY PATCH to buy RUBY FOR FREE !!!!
its work for me
1. Just install Lucky Patcher you can search n google
2. now open Lucky Patcher and search your djinn caster
3. Open Menu Of Patches > Created Modified APK FILE
4. and choose APK Rebuild InApp And LVL Emulation
5. Wait until succeed, after succeed open your modified APK from Lucky Patcher and install/overwrite it
6. Open your Djinn Caster and try purchase RUBY ENJOY !!!!
{KEMCO} Djinn Caster - Premium APK + Modded Save File {Root Needed} was released by Axiom and 62001 people like you already found this fantastic release! Find this and over 20,000 unique Android Games & Apps like this at sbenny.com, here since 2014 to serve the best Gaming Community in the world with hundreds new, safe and amazing releases every day.