FDR and the Western allies had two options they could pursue:
1. Just keep advancing and keep any territory they overran, thus pissing off the Russians and probably sparking WW3 on the heels of WW2
2. Or give the Russians a rather reasonable slice of Europe and thus adverting WW3
A "reasonable slice" would have been well to the East of the Elbe, and likely, well to the East of the Oder. Remember, Stalin started the war virtually on Hitler's side.