It depends. Russia actually has quite close cooperation with Chinese nuclear industry. One example is Russian gas centrifuge technology used for uranium enrichment in China. I kind of doubt they would sell their latest technology to China. But they might sell the Yasen or Borei design once the Laika submarine gets close to operation. I could see a sale happening in a couple of years. I do not think China will need a deal like this however.
If it was just a submarine lease similar to the one they have with India then Russia could it right now I think. This would make a lot more sense, I think, since it would allow China to evaluate Russian submarine technology hands on. And they would basically be helping Russia to expand its submarine production which would help offset increase in size of US allied nuclear submarine fleet.