There was a bunch of unused/underutilized office space after much of the federal workforce went remote. My understanding is that they have mostly sold off or rented out the unused space, though.

Undisciplined

econ

Four Ways to Unrig the Financial Economy (Wall Street Journal, Elizabeth Warren)

denlillaapan

no clue. I'm not aware of any "unused federal property" unless it's the vast landholding that's already being discussed. I doubt that there are, like, office buildings in D.C. not in use. 

Maybe sell Congress? It's a pretty building with some tourist promise, I believe, but very little valuable activity