Considering it was only "manchurian" not han land for a few decades i think it is very fair do you not? Show me which actual Chinese dynasty from Xia to Ming did China dominate Siberia/Outer Manchuria and sent Han to populate those areas. Show me historical evidence that Han people ever migrated or used outer mongolia/manchuria besides Manchurians occupying it for a few years then handing it to the Russians.
I can only let the Chinese speak for me on this one. If they accept it was never really Chinese thats fine.
Sakhalin island, a small island off the coast of outer Manchuria, (size of Austria) was under Chinese sovereignty for 500 years and now it's with the rightful owners, the Russians. Who have controlled it for about 75 years. The true historical owners with a 75 years history there.