There are two locations where you can exchange your voucher for a ticket in Sendai as follows
SENDAI STATION EAST EXIT JR BUS INFORMATION STATION
Business hours: 6:50AM-7:30PM
MIYAGI KOTSU MIYAKO SENDAI HIGHWAY BUS CENTER
Business hours: 6:50AM-9:00PM
Also, please check other designated locations where yon exchange your voucher for a TOHOKU HIGHWAY BUS TICKET at
Exchange site