To further investigate these matters, the Oversight Committee subpoenaed Mazars for Trump's and his business financials  from 2011 to present.  Trump then sued Mazars asking the DC District Court to declare the subpoena invalid and unenforceable. The Oversight Committee intervened to defend its subpoena.

The District Court found the subpoena to be valid and that Mazar's must comply with it. Then Trump appealed to the DC Circuit.

On October 11, the DC Circuit affirmed the District Court's order and held that Mazar's must comply with the subpoena. The Circuit Court held that the Committee was engaged in a "legitimate legislative investigation" on a subject on which Congress could propose legislation.  Judge Neomi Rao's dissent, essentially, inserted itself into the legislative prerogatives and, is considered by some, to be an embarrassing dissent (Rao was appointed, by Trump, to the Brett Kavanaugh seat).

There is a long history of legislative investigations; as early as 1792, Congress asked for records from President Washington as part of an investigation, and many congressional investigations have followed over the years.  They were not necessarily a part of an impeachment inquiry or process, nor were they deployed for impeachment purposes.

Moreover, the Supreme Court has affirmed Congress's broad power to investigate, for example, administration of existing laws as well as possibly needed laws. Following information that Trump might have failed to disclose certain financial holdings in required disclosures, Congress would have an interest in obtaining more facts so as to inform the need to strengthen existing financial disclosure laws and consider the need for additional restrictions on presidential conflicts of interest.