According to Callis, the two sides agreed on a signing bonus worth $1.3 million, below the slot value of $1,711,900 for the No. 36 overall pick.
Mountcastle, a shortstop out of Hagerty High School in Florida who projects as a future third baseman, was a 2015 Perfect Game 1st Team All-American.
Joey Nowak is a reporter for MLB.com. Follow him on Twitter at @joeynowak. This story was not subject to the approval of Major League Baseball or its clubs.
