Do you have any other customers? Or is this your first customer? If it is your first customer, you want to sell them on being a beta client and the fact that they will get input onto what your customer should do.
Beware that you need to find the "right" beta client. You don't want someone who is a perfectionist or finicky because they will be upset when they find bugs or you can't build them a feature. Most potential beta clients will be people who like to be on the cutting edge of technology and trying new things.
The best way to see if someone is good fit is to show them a demo of what you have so far, see if they are extremely enthusiastic about it, and ask them as many questions as possible to see if they are okay with buggy software.
I've also heard many people say that you should give your product away to get your first couple clients. I'm a big believer in charging from the start, so try to get them to pay for your half built software. If you're solving a real pain for them, they will gladly pay for it. Then make them happy, keep improving your software, and you should have a good base of referencable clients pretty quickly.