[4CJ]

In State Of Ga. v. City Of Chattanooga, 264 US 472, (1924), Justice Butler wrote "[t]he power of the city to condemn does not depend upon the consent or suability of the owner," even when the property was owned by a sister state.

But you are requsting a case where Federal property was taken via eminent domain without the prior approval of the federal government.

The Illinois legislature chartered a railroad and gave it eminent domain powers, the "power to build, maintain and use a railroad bridge over the Mississippi River, or that portion within the jurisdiction of the State of Illinois, at or near Rock Island."  Rock Island had housed Ft. Armstrong, which had been abandoned in 1836.   The railroad took part of Rock Island for a bridge, even though it was federal property.  In United States v. Railroad Bridge Co., 6 McLean 517, (1855), Justice McLean wrote that when "the site is abandoned, the reserve falls back into the mass of the public lands."  The railroad won.  

[Non-Sequitur]

Had Fort Armstrong not been abandoned then it might have made a difference in the Justice's decision. Sumter wasn't abandoned.

[Non-Sequitur]

Another thought, how does this apply to property owned by a foreign government? Would the District of Columbia be allowed to condemn and seize the embassy of, say, Iraq for a city park? Can Chicago condemn the Saudi consulate? In the case of Sumter, from the southern position it was foreign property since they no longer considered themselves part of the United States. And they simply seized it without any legal proceedings at all, starting a war in the process.