I don't believe it's available at this time if it isn't in the graph api which would seem to be the case based on what I've read (or haven't read as there's no mention of it after the credits "promotional period" ended).
I just tried using the users.getStandardInfo method and received this error: The underlying FQL query made by this API call has encountered the following error: credit_balance is not a member of the user table.
Returned by this:
$obj = json_decode(
curl_call_fb(
'https://api.facebook.com/method/users.getStandardinfo'.
'?uids='.$user->getFacebookId().
'&fields=credit_balance'.
'&access_token='.$access_token.
'&format=json'
)
);