I once had an experience that I gave a sample work for a client, after sometime I saw my writings already posted on the client site.
The best practice is to never provide new, original pieces as samples AND ask for 50% of the project up front. If a new client wants a usable sample, they should pay you for it.
You can send the sample as an image file instead of a .doc or .pdf format. If the potential client is desperate enough to re-type the entire article manually from an image, then kudos to him. But being in this industry, it's pretty common to come across 'bad' clients like that. Now that you know someone's stolen content for you, holster your guns and fight for your rights; if you're up for it. Here's an example of what you could still do to solve this issue. You could publicize the sample that you wrote on your own website. This is so that any SEO value that the thief could gain from stealing your content will be nullified, since Google will find duplicate content. The article would then do more harm than good to his website. After that, you could drop him a message notifying him of the action that you've taken and offer him three choices: make payment, give credit to your services at the bottom of the article, or simply take it down. It's not a fool-proof method, but it doesn't take much effort and it's worth a shot
The truth is that a good client will rarely if ever ask for a free active sample. In fact, I recently offered to make one for a client and they refused. Never give it away. I don't care how enticing the fee is. My years of experience have shown that the ones who want an up front just to "make sure" you can do it - are the ones that almost never close, and if they do, you end up wishing they didn't. The first two responses are great advice.
Thank you all for your advice.. Nice to get great advices from people who is an expert in the industry. We did nothing with the client all we can think of is karma is now digital. May the nature finds its revenge by its own self.