I’m guessing that AG Barr is not required to answer Nadler.
As House Judiciary chair, Nadler does have the authority to conduct oversight and compliance of the DOJ to make sure the department is operating within applicable laws and spending its budget appropriately.
That said, the committee doesn't get to direct the operations of the Department, or interfere in them. Nadler can get information about this "channel" and find out that there is a deputy attorney general or US attorney who is overseeing the intake of information, establishing a chain of custody, and a procedure or procedures for evaluating the information gained. Is a dossier of information assembled by an untrustworthy source being used without independent confirmation as a basis for a counterintelligence fisa warrant targeting an American on American soil? That's the information Nadler and his committee are entitled to receive.
The actual information, how it has been evaluated, where it came from, and who it's about are part of an ongoing investigation. Revealing the information before the investigation is complete would impede any potential prosecution and impact the rights of the possible targets of a presumption of innocence.