You don't have to pay an exempt employee for any workweek in which he or she performs no work. It must be a full workweek. If the exempt employee performs any work (say 1 hour), you would have to pay them the full salary or put their exempt status at risk.
"There is no requirement that the predetermined salary be paid if the employee performs no work for an entire workweek. Deductions may not be made from the employee’s predetermined salary for absences occasioned by the employer or by the operating requirements of the business. If the employee is ready, willing, and able to work, deductions may not be made for time when work is not available."
http://www.dol.gov/esa/WHD/flsa/FurloughFAQ.pdf