The Problem with your proposal for a negotiated deal is Wolf, that Yasen is pure Russian. If the PLAN got there hands on it its weapons and systems would tie its operations to Russia. The Chinese have been moving away from Russian or depending on anyone, the PLA as I see it has a deep seeded projectionist impulse. If they can get it at home then they will, if they can't then they will get the resources to. If it takes a long time then they will use a interim import until they can get it in the PRC from PRC owned or closely related maker. Yasen being a Russia sub uses Russian type torpedoes, not the Chinese spec and the Russian torpedo specs are also different now, she loads Russian Cruise missiles, not the types used by the PLAN. She would basically drag China back to Russia. What the PLAN would really want from Yasen is her silence. Her Pump Jet, her reactors, her mountings and sound dampening insulation, parts of her sonar and maybe her Hull metallurgy. They want her technology's not her hull. Her hull would be a drag. Her technology would allow the PLAN to make there own Yasen. Without the Russians.
as to the Deal or no deal China is catching up, and its needs are expanding. The S400 is a fair option, and Putin is making this offer to try and get China back interested in Russian industry with the Hopes that he can get some more Chinese investment. Look at the other pushes we have heard of from Russia to China, Russia trying to get deals in civilian aviation by offering to joint for a regional jet program, the Russian offer of Su35 which yes was never taken but there must have been a offer. These are sweeteners for the PRC a way for Moscow to say "Hay Beijing, listen we know you love our Oil but how about taking a look at our other fine products..." Russia is getting a little bit jealous, they see RMB flowing into emerging Africa and are not seeing the same for them. Though India is buying there recent buys have included American and now French Hardware. Venezuela has bought alot but its just not anyway enough to keep the Rubbles greasing the Russian economic machine especially when Venezuelan dollar is getting less and less worth. The Saudis just paid for boat loads of toys as gifts for Egypt but Putin knows that that was a move by them to get the US's attentions'. The War in Syria is making a buck but the Russians know Assad is more and more a puppet for Iran and more and more importantly bankrupted as the war keeps going all the weapons Putin sends south is more and more a IOU. Europe is buying fuel but with the political situation in the Ukraine they know they are going to have to find another supplier. We often don't think of it like this but Russia is a emerging market nation. The fall of the Soviets way back when followed by the collapse of the Rubble in the 1990s pretty much dropped them back into the stone age for a decade. Now Russian is moving back up but its unstable there exports are limited and specialized. Russia sees the PRC who was once reliant on Russia and is more or less trying to cash in on there previous good relationship.
Now as for Russia buying from China, the two nations have different ideas in defense and motivation. China has come a long way but Russia's military and industrial development are going another. Russian client states and Chinese Client states are also a issue. They don't always get along and that can cause backlash.