Havertz is a top target for Chelsea.
"We have an agreement - if it fits, he can go this summer," Voller told Bild.
"As of now, he is a player of ours. We have our ideas, we know what he can do. I personally hope that he stays one more year.
"Of course, you can't force anyone. Havertz knows what he owes to his colleagues, the fans and the club."