You fail to realize that your original premise is nothing but crap. There is no law that says the state department is the only entity that can conduct foreign policy. A president can chose whomever they want to conduct foreign policy, including negotiating with countries that we have a senate ratified treaty, intended to root out corruption... like Biden's.
Or were you unaware of this treaty?
Treaty with Ukraine on Mutual Legal Assistance in Criminal Matters, ratified by the U.S. Senate on Oct. 18, 2000.
https://www.congress.gov/treaty-document/106th-congress/16/document-text?overview=closed
https://www.congress.gov/106/cdoc/tdoc16/CDOC-106tdoc16.pdf