You all are looking at this from the wrong perspective, ie number crunching. Stalin had this many divisions, he had this much manpower left, and this many tanks. Irrelevant.
Stalin was a paranoid f*ck to the point of madness. Make him believe that the Western Allies are about to strike a peace deal with Nazi-Germany and he will piss himself and rush to give away a peace deal to Germany. The really, really hard part would be to come up with a plausible explanation to make Hitler sign it.
How to make Stalin believe that Nazi-Germany and the Western Allies is about to strike a deal? Let one of his spies in Madrid have thought he saw the German and British ambassador eat lunch with each other or something equally stupid, it would drive Stalins paranoia into overgear.
How to make Hitler accept a peace deal? Let Hitler have a moment of brief sanity? Ok, that's ASB, sorry. Well make him come up with a new plan that this would get him the time his Wunderwaffen needed to kick the Western Allies off the continent so he could deal with Stalin later, or something equaly stupid, it all depends on how it is written.
But IMHO it is not ASB for Stalin to sue for peace even after Bagration, IF it is written well enough and IF there is a reasonable explanation (Stalins paranoia should be a sufficent reason). Hitler is the tougher part, but IF written well enough and given sufficently reasonable explanation, Hitler might accept a peace deal. Of course neither would trust each other, and a new war would very likely erupt before 1950.