In time, they may retake Outer Manchuria the same way they retook Hong Kong.
Outer Manchuria is in light red on this map. Some also consider the island of Sakhalin to be part of Outer Manchuria.
What do you mean, "the same way they retook Hong Kong?" The United Kingdom had a contract with China regarding Hong Kong, and the City reverted to China in 1997 as part of that contract. For better or worse, the U.K. voluntarily ceded Hong Kong back to mainland China as part of an agreement struck a century before. Is there such an agreement between China and Russia?