China knows that it has very strong hand and is most likely pushing for big discounts.Alexey Miller, the Gazprom CEO, is hyperventilating. Power of Siberia, even when it gets to full capacity, is roughly equivalent to the Nord Stream pipeline. Europe also consumed gas from the Yamal pipeline over Poland and the pipelines over Ukraine like the Brotherhood pipeline. Gazprom has lost a huge amount of foreign sales which cannot be covered by Power of Siberia alone.
Then there is the fact that the gas which used to be sold to Europe came from the deposits in the Yamal peninsula in the Arctic, while Power of Siberia uses gas from the Far East in Siberia. Right now, most of the gas that used to be sold to Europe has nowhere to go. Russia has been selling it to neighbouring countries like Azerbaijan, Kazkhstan, etc but they cannot replace European demand.
In the long term to sell the Yamal gas to China will require the construction of the Power of Siberia 2 pipeline and Arctic LNG 2 facilities.
Russia is resisting this. Dunno what they are waiting though because with each passing day China's hand on this negotiation gets stronger and stronger while Russia's weakens.
For certain, I wouldn't want to be in Russia's shoes trying to negotiate such a deal. At this point you might as well as take the L and move as fast as possible on construction of the facilities and infrastructure. These things take a long time to be done even after the deal is signed