One thing you could do is add a page to your website that you link to from your Facebook page but not in your website. Think of it has a hidden page only for people coming from Facebook. On that page, you could add the Contact Us form and some text explaining how to inquire and get the special Facebook deal. Here's an example of what that might look like. Note, all messages sent through this form show up in your "Other" messages and not your "Inquiries".