Q: Was Koenigsberg area given to the USSR for 50-year administration rather than permanently?
This is partially correct insofar that neither at the Potsdam conference nor in any agreement before the Soviet Union 'was given' the Königsberg area as permanent possession. While Stalin really 'wanted' that area as 'legally part of the Soviet Union' for geo-strategic reasons (ice free port) and out of 'revenge' and compensation for the war, no other nation at the time agreed to fulfill that desire in an internationally legally binding form.
As the Potsdam Declaration indeed states clearly:
VI. CITY OF KOENIGSBERG AND THE ADJACENT AREA
The Conference examined a proposal by the Soviet Government that pending the final determination of territorial questions at the peace settlement the section of the western frontier of the Union of Soviet Socialist Republics which is adjacent to the Baltic Sea should pass from a point on the eastern shore of the Bay of Danzig to the east, north of Braunsberg and Goldap, to the meeting point of the frontiers of Lithuania, the Polish Republic and East Prussia.
The Conference has agreed in principle to the proposal of the Soviet Government concerning the ultimate transfer to the Soviet Union of the city of Koenigsberg and the area adjacent to it as described above, subject to expert examination of the actual frontier.
The President of the United States and the British Prime Minister have declared that they will support the proposal of the Conference at the forthcoming peace settlement.
As a reading aid, Wikipedia translates the above as:
City of Königsberg and the adjacent area (then East Prussia, now Kaliningrad Oblast).
The United States and Britain declared that they would support the transfer of Königsberg and the adjacent area to the Soviet Union at the peace conference.
Of course, there is nowhere any mention of a fixed "timetable" nor any "50 years", since at the time a peace settlement was envisioned as 'coming very soon'. That part of the claim is nothing but a conflation or invention.
The actual text was quite disputed and for example in the final form the Russian version diverges from what the English reading world usually reads:
"The Conference has agreed in principle to the proposal of the Soviet Government concerning the ultimate transfer" The Soviet text reads, in literal translation, “concerning the transfer”
— Foreign Relations of the United States: Diplomatic Papers, The Conference of Berlin (The Potsdam Conference), 1945, Volume II, L/T Files, No. 1383 Protocol of the Proceedings of the Berlin Conference
That the text is imprecise in that regard is obvious. But under legal examination it opens up for interpretations. With the notable exception of the USSR no one else saw the agreements reached at Potsdam towards Eastern German territories as 'final' and 'permanent'. This is also evidenced by the US to continue strongly opposing the incorporation of the Baltic states into the USSR.
Article IX concerning the areas of East Prussia ceded to Poland spell out more clearly the nature of the Soviet control of Königsberg, referring to it as having been
"placed under the administration of the Union of Soviet Socialist Republics in accordance with the understanding reached at this Conference."25
Thus, Königsberg was neither appended outright to the Soviet Union nor was it to be considered part of the Soviet zone of occupation, which had been outlined earlier in the agreement.
It can thus be seen that while eventual Soviet sovereignty over Königsberg was anticipated, it was also expected that the peace settlement to finalize this was to have been rather shortly forthcoming. The subsequent development of the Cold War and the creation of the Federal Republic of Germany and the German Democratic Republic as separate states precluded the signing of the peace treaty until 1990. In the intervening period, the German Reich continued its existence de jure, although the state of Prussia itself was abolished by decree of the Allied Control Council in 1947. Thus, technically, the Soviets may never have held legal title to territory.
— Raymond A. Smith: "The Status of the Kaliningrad Oblast under International Law", Lituanus, Lithuanian Quarterly Journal of Arts and Sciences, Volume 38, No.1 - Spring 1992.
The Soviets then went ahead and made the situation a fait accompli. Interpreting "administration" as practically "annexation". During the Cold War West Germany upheld the claim to these territories officially until 1970, and subsections of the political spectrum at least until 1990. Fringe groups claim of course to this very day an irredentist perspective. Interestingly, the Soviet side during the negotiations in 1989/1990 seems to have been open about transferring back this exclave to Germany. The official German side declined. But the legal status of that territory was indeed only finalised in 1990 by all official sides directly involved expressing "this is fine, now".
Summarised in this introductory textbook:
Between 1945 and 1949, Germany's conquerors reduced the size o f its territory,[…] Consistent with the Allied agreements at Yalta, those German provinces east of the Oder and (western or Lausitzer) Neisse rivers (the Oder-Neisse line), including East Prussia, Silesia, and most of Pomerania—which totaled approximately 25 percent of Germany's prewar territory, were put under "temporary" Soviet or Polish administration. The ultimate fate of these eastern territories was to be decided by a final peace treaty between Germany and the wartime Allies. However, such a treaty was never signed because of the Cold War. In 1970 provisionally and in 1990 unequivocally, following the accession of East Germany into the Federal Republic, treaties with the Soviet Union and Poland recognized these losses as permanent.
– Eric Langenbacher & David P. Conradt: "The German Polity", Rowman & Littlefield: Lanham, Boulder, 112017.