He's basically asking people to write him a 500 words article for free as a "sample".
Let me get this right: So to work with this person, they ask you to write them a sample article of 500 words. That's supposed to be a test to determine your quality as a writer, right? But in reality, that sample article given on a specific topic can be exactly what OP wants, and they don't need your article writing services at all after that.
To make it more believable, OP could trick you with big promises that they will throw a lot of work your way etc., etc. Be careful who you work with, especially on platforms where evidence could be easily deleted such as Telegram. I am not saying it's a scam, but it could play out exactly as I explained above. Take some time to check out OP's
trust feedback as well. Although it's not from DT members, it should still be taken into consideration.
Btw, what does OP want their clients to write about?