It was presumably a piece of collector's memorabilia brought up by a Chinese taikonaut only to be abandoned after the station was evacuated.
The Chinese coin was presumably destroyed upon re-entry. If not, the coin was buried or submerged in water after hitting the ground.
It is unknown what dynasty this coin was produced in because we see it only for a few seconds and it is small. However, judging by the wear on the surface, texture, thickness and the logic that the Chinese would have access to their own coins and wouldn't bring replicas into space, the ancient Chinese coin was probably a CGI replica of an authentic ancient coin. The coin was from later in ancient Chinese history, probably from the Song, Tang or Qing dynasties. It is mostlikely Qing judging by how many they made and the size and color of the coin.