Matter of Diaz-Ruacho, 24 I. & N. Dec. 47 (BIA 2006) (noncitizen granted voluntary departure, who subsequently fails to leave and fails to post the voluntary departure bond required by INA 240B(b)(3), 8 U.S.C. 1229c(b)(3), is not subject to penalties for failure to depart within the time period specified for voluntary departure, since "posting of a voluntary departure bond is a condition precedent to permission to depart voluntarily. . .").
http://www.usdoj.gov/eoir/vll/intdec/vol24/3546.pdf