The purchasing email isn't mine
How do I change the purchasing email to my own Google accounts so that I can spend my money.
Solution 1:
From Android Enthusiasts: How do I change my email address on Play Store as the one I have is no longer in use:
Go to settings->accounts->add account->and add another Google account with the new email. Then go into the play store and there will be your email with a drop down arrow. Tap it and select the new email.
Pokémon Go Android will always lead you to the Account which is logged into PlayStore when you intend to purchase the In-Game curreny "Gold Coins".