Only the President can grant CBA...and if he doesn't, nothing can be done.
Here's another little tidbit about Congress granting immunity from prosecution to witness for their testimony (might be worth remembering, just in case.;-)...
Congressional committees have the power to grant testimonial immunity to witnesses who testify before members of Congress. Congressional investigations into allegations of misconductsuch as the Watergate investigations in the 1970s and the iran-contra investigations in the 1980srely heavily on witness testimony. Whereas prosecutors simply decide whether to grant immunity to a witness, congressional committees must follow more formal procedures. Immunity may be granted only after a two-thirds majority vote by members of the committee. Ten days before the immunized testimony is given, the committee must advise the Justice Department or the Independent Counsel of its intention to grant immunity.Source: free (legal) dictionary
WHY? What was more important in Benghazi than Americans? Something sure was because Obama did not want it destroyed!