It’s actually required by a treaty which got the Consent of the Senate:
https://www.state.gov/documents/organization/123676.pdf
Russia (and the US) can ask for this process at any time, it’s not something that Trump can refuse.
McFaul is well aware that he can be subpoened to testify, and he seems to be worried about what might be exposed.
The “Treaty between the Unites States and the Russian Federation on mutual Legal Assistance in Criminal Legal Matters” was a treaty proposed by President William J. Clinton. It was signed in Moscow June 17, 1999.
President Clinton and Secretary of State Strobe Talbot submitted the treaty to the Senate in Feb. 2000 for ratification. By unanimous consent the Senate led by Democrat Senator Tom Daschle agreed to take up ratification. Joe Biden (D-Delaware) chairman of the Foreign Relations “Committee rammed it through his committee, and from there it went to the entire Senate for ratification. The treaty was ratified in December 2001.
The treaty was signed during the President George W. Bush Administration. Secretary of State Colin Powell and Russian Ambassador to the U.S. Yurly Ushakov signed the treaty on Jan. 31,2002.