Easy, the Soviets put aside thier German-hate for a minute and decide that a single bigger nation would be a greater buffer between them and the west then 2 or more smaller nations...so they say 'fuck the poles' and gives East Germany all of it's Pre-1914 Eastern Territory, with the exception of any part of that that is Russian...the Part of East Prussia that Russia took may or may not be given to 'The Peoples Democratic Republic of Prussia'... whatever is left is given to the poles or annexed directly into the Soviet Union...Ethnic Clensing to make the areas German again would probobly be undertaken to make Prussia more powerfull and better able to handle it's western neighbors.
when the Berlin Wall finally comes down, Prussia is in a place to negotiate it's place in the post Soviet world...so it says that it will reintegrate with Germany, but on the condition that any and all Current Prussian Territory is given to Germany....and thusly Germany is given it's previos territory...
andouther way to go about this is to change the Pre-1914 with Pre-1939...that would be more believable, but it'd be less powerfull, and thusly it'd be harder to justify the russians doing it.
Neither of these are plausible. The first requires that Stalin ignore what the Nazis did in Russia for the past 5 years. Stalin without a doubt would prefer to have territories populated by Poles to populated by Germans, and the Polish territory OTL is what the Poles could occupy successfully without overstretching. In particular, you're not seeing Germany get 1913 borders again. Ain't happening. Nor even 1938, given all the crap that East Prussia being separated caused. At best, you might see Germany keep Hither Pomerania and Silesia, which was on the table until the Poles traded Konigsberg to the USSR in exchange for Stettin. And that they needed a good industrial region.
And by the Berlin Wall falling, those territories are so thoroughly Polish that any return to Germany is ASB.