Is 999 a kind of slang?

A client who I was negociating an art piece said me this in the chat. The context was whether or not the art should have a background. He said it wasn't necessary and then said this:

"I prefer composition to gratuity. A picture is worth a 1000 words. If you can do 999, that is fine too."

I really didn't get the "999" part. A quick online search didn't give me clear answers. I 'll be very grateful if someone could give me a little help.

Thanks in advance! ^^

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Redseahorse 4 38 589
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Redseahorse 4 38 589
Bonus tip:

"A client WITH WHOM I was negociating..."