Ahead of the Judiciary Committee hearing, Mr. Benczkowski told the panel that he had previously been forbidden by his firm’s confidentiality agreement from disclosing his work for Alfa Bank, but had obtained a waiver.

The disclosure was made in a July 19 letter to the committee that was obtained by The New York Times.

“Your staff has indicated that the committee may wish to question me at my hearing regarding the fact and scope of my work for Alfa Bank,” Mr. Benczkowski wrote, saying his waiver would permit him “to discuss the fact and scope of the representation at the hearing. As I am sure you understand, ethical considerations prohibit me from disclosing confidential legal advice or any other information protected by the attorney-client privilege under any circumstances.”

Mr. Benczkowski’s law firm has offices around the world, and it represents many foreign clients. But his association with Alfa Bank in particular could be awkward in this period of heightened scrutiny on ties between Mr. Trump’s associates and Russia.

Mr. Benczkowski’s disclosure comes at a time of tremendous political and legal scrutiny over those ties, including a special counsel investigation into Russia’s interference in the 2016 election and whether the Trump campaign colluded in that effort.