Are tou sure you bought 500 not 1000 Diamond?
Please check again.If its 500 diamond please contact in game customer support
Tap on the blue headset icon on the top left corner besides google play games or youtube icon
Go to help>payment problem
Get the payment issue feedback form
Fill up all the information and the screenshot of the money receipt and describe the issue.
Submit
Please wait 3-9 days for feedback depending on the traffic .