That's on FDR. What's done is done.
GERMANY herself had no way of changing the historical outcome. That is what you asked. If you rephrased this as "what is the best deal the WAllies could have got with regards to the eastern German borders" then that's an entirely different question.
Germany was impotent.
Actually, FDR and Churchill did
not accept the Oder-western Neisse line at Yalta. (Incidentally, Stalin's enthusiasm for this more westerly line was a recent development; throughout 1943 and 1944 he had found the Oder boundary sufficient compensation for Poland.) For that reason, the declaration adopted at Yalta spoke vaguely of Poland getting "substantial accessions of territory" instead of giving specific boundaries.
http://avalon.law.yale.edu/wwii/yalta.asp
(As Herbert Feis notes, "The Russians had not given up without a last try. Molotov had urged that the second sentence read: "It is recognized that Poland must receive substantial accessions of territory in the North and West,
with the return to Poland of her ancient frontiers in East Prussia and on the Oder. Roosevelt asked how long since these lands had been Polish. Molotov said very long ago, but they
had once been Polish. "President (laughing to Prime Minister): Perhaps you would want us back?" Prime Minister: Well you might be as indigestible for us as it might be for the Poles if they took too much German territory." Stalin withdrew the proposal."
https://books.google.com/books?id=CQzWCgAAQBAJ&pg=PA525 Molotov was of course correct; all Silesia had belonged to Poland--prior to the fourteenth century...)
What happened is that after Yalta the USSR unilaterally turned the territory east of the Oder-western Neisse line over to the Poles. At Potsdam the US briefing book stated that the US should be prepared to accept "with reluctance" Polish claims to the Oder-eastern Neisse line but not beyond. (The difference between the Oder-eastern Neisse line and the Oder-western Neisse line was not trivial, involving an area of 8,100 square miles with a pre-war German population of 2.7 million.
https://books.google.com/books?id=NQHSxnDN0_gC&pg=PA115) But Stalin stood firm, and given the
fait accompli (the Germans had already been driven out and the territory turned over to the Poles) there wasn't much the Western Allies could have done about it by this point unless they wanted to start World War III. Even so, the Potsdam Declaration maintained the fiction that the Oder-western Neisse line was merely the
provisional German-Polish border, and that the final border would be determined by a peace conference:
"...The three Heads of Government reaffirm their opinion that the final delimitation of the western frontier of Poland should await the peace settlement.
"The three Heads of Government agree that,
pending the final determination of Poland's western frontier, the former German territories east of a line running from the Baltic Sea immediately west of Swinamunde, and thence along the Oder River to the confluence of the western Neisse River and along the Western Neisse to the Czechoslovak frontier, including that portion of East Prussia not placed under the administration of the Union of Soviet Socialist Republics in accordance with the understanding reached at this conference and including the area of the former free city of Danzig, shall be under the administration of the Polish State and for such purposes should not be considered as part of the Soviet zone of occupation in Germany." [my emphasis--DT]
http://avalon.law.yale.edu/20th_century/decade17.asp
This was of course pretty much a fig leaf, but again, it is hard to see what more the US and UK could have done (short of going to war) given the "facts on the ground."
So the answer to whether Germany could have gotten better borders with Poland in 1945 is basically "only if Stalin wanted them." He had the troops there, after all.