I would explain it like this:
“All I need from you, Mr. Client, is your authorization that, should you hire this candidate, you will pay us our fee. For this type of presentation to you, there is no retainer up front. I’m waiving that. If you don’t feel the candidate is worth the fee, then don’t hire him.”
If they still insist that you show them a candidate’s resume before gaining their agreement or that they want to ‘try’ your service first without agreeing to pay you for it, then that is a clear indication that the client is a problem client. If they are a problem client right now at this stage of the process, then they will be a problem later on in the process and in other areas. They might slow pay you or pay only part of the fee, that sort of thing. From now on, don’t worry about clients who do this. That just means they are risky clients, and you should stay away from them.
More posts on my blog: http://scottlove.wordpress.com.