If you are asking with reference to a freelancing gig. I would say only one word. "AVOID!"
As a matter of fact, I just declined the same request by a client this morning. It is just a wastage of your precious time and resources to write an unpaid sample.
On the other hand, use a portfolio type website (like Hubpages) to showcase your written content. If the client is not even bothered to pay a visit to your given link, how do he suppose us to write an unpaid sample article?