In short, before I refunded it, I investigated OP's wallet txs. OP previously had a tx output to a destination address that contained the first and last character digits that identical to mine (a rare coincidence).
As an illustration:
A1b2Cdfdruirj4grhg2urh5ggriorjf3D4e5F : belongs to OP
A1B2c1jijrj5ofmmf8jkfidk0mawq3d4E5F : mine
That's an interesting fact. But I am still confused about how he copied your wallet address instead of his wallet. After reading your post, I guess I always did it wrong. I always check the first and the last few characters to ensure where I am sending it. What is the odd of copying a similar address? No matter how low the chance is, a single transaction can cost me a thousand dollars.
Op was lucky that he sent the amount to you, and he was able to contact you within the forum. Almost 99% of Bitcoin address are anonymous and we don't know the owner of those addresses. So it's impossible to contact the owner to get the money back.