[SJackson]

Minor difference, I think you’re thinking of William W. Belknap, Grant’s Secy of War. But yes, impeachment and trial took place after his resignation. Not entirely sure that’s a precedent though. The propriety of a trial after out of the administration never went before a court, and he was considered guilty by most, but virtually all of the Senators voting not guilty indicated they did so because they had no jurisdiction over a former employee rather than because he was innocent.

[Bubba Ho-Tep]

Thanks for the correction on which of the many corrupt officials in Grant's administration was impeached. As far as the legal propriety, I'm not sure a court would touch it. They've said in the past that impeachment was a political matter, not a legal one, and not subject to judicial review-- "nonjusticiable" in legal terms. See the unanimous SCOTUS decision in Nixon v. US, 1993

https://www.law.cornell.edu/supremecourt/text/506/224