Robert Chapman tells CNBC's Scott Wapner he shorted ADP shares with an average cost of $119.50. : The short-seller Robert Chapman and hedge-fund billionaire Bill Ackman are facing off again over a stock. Chapman told CNBC's Scott Wapner he shorted ADP shares, a bearish bet. The founder of Chapman Capital didn't disclose the size of the trade, at an average cost of $119.50, but it sets him up opposite Bill Ackman's Pershing Square, which recently bought an 8 percent stake in ADP and nominated three directors to the company's board.