As a variant, you can check whether it(play billing library dialog) is a separate activity or just a dialog. After the dialog is closed an appropriate callback method(onStart in case of activity) will be called in your activity/fragment.

If I have understood correctly, every time the billing flow is launched and afterwards the dialog is closed, you wanna run some code accordingly. if so, every time the dialog is closed, there is a reason. maybe it has been canceled by the user, an error occurred or maybe the purchase was complete and successful. In all of these cases the onPurchasesUpdated() is triggered with a specific BillingResponseCode which you can check and run your code accordingly. here is a list of all the possible response codes.