China Rejects the Convention!
The decision can be taken by Russia, if Manchuria is sphered by Russia. That gives China the event The Convention of Peking, in which they can accept the convention and secede Outer Manchuria to Russia or reject it. This event happens, if they choose to reject it.
The only option for Russia gives them an 'acquire state' casus belli, reduces relations and influence with China by 200 (moving China from Sphered to Neutral or from Friendly to hostile) and releases China as a puppet, if they have been puppeted before.