Patel v. Gonzales, 470 F.3d 216 (6th Cir. Dec. 4, 2006) (where BIA rejects underlying reason for an IJ's decision to deny voluntary departure, but makes no determination of its own on the issue, BIA has not exercised discretion, requiring remand for a ruling by the BIA on whether petitioner was entitled to voluntary departure).
http://caselaw.lp.findlaw.com/data2/circs/6th/063197p.pdf