I know Dortmund is a good team but I am not sure if this is the right move for him because Sunderland just got promoted to the premier league and making appearance in the premier league will make him exposed and easily catch the eyes of bigger teams instead of moving to Germany.
Wherever he plays, if he can improve his skills then he will be in the spotlight and don't forget Bundesliga is one of the top leagues
Dortmund are planning a long-term investment in this player as they have been doing for the past few years.