Preventing Facebook Social Login if email found

My site has two types of users a business and regular(non-business). The regular can also login via Facebook social login.

Consider the following flow: A business user registers first and then the same user goes to create a regular user account w Facebook Social login... this is the scenario I am trying to prevent.

I want to prevent this by checking the email and if this allready exists.

So consider this UX path and tell me what you think

  1. The Business User clicks the FB login button...the auth dialog appears.
  2. With FB.API call I get his email
  3. On clicking OK I make an ajax request and check for the e-mail in the DB.
  4. If found prevent registration and call FB.logout to log him out of FB.

Ideally i must check the email before the popup appearing...nonetheless I have not found a way getting the email from FB,API before a connection is made to FB...something that will inevitably bring up the dialogue...

What do you think?