I agree with Charlene, you can't really bill for this. Basically the dr is donating a couple of minutes of his time in order to obtain a new patient, who will hopefully become long term. Look at it as an advertising expense. As Charlene said advise the person that you can't actually schedule an appt, but that he can come in and the dr will take a couple of minutes between patients to speak to him, but it may be a wait. Of course if you make him wait too long that might make him decide not to come there.
Michele