Having established himself as the club's key man at the age of 20, Havertz is being heavily linked with a switch to a bigger club.
Liverpool are allegedly prepared to pay in excess of £100m for the playmaker, and Voller has acknowledged that the Bundesliga outfit will consider any offers.
Speaking on 100% Bundesliga – Football at Nitro, Voller said: "I am glad that we still held [on to Havertz] this year. We'll see how it looks in summer. He still has a contract until 2022 - these are, of course, the classic facts.
"There was already interest in the summer, but we told him that it would be good for him to stay with us for another year. And that was the right decision."
Havertz, who is also interesting Bayern Munich, has contributed five goals and two assists from 19 outings in Germany's top flight this season.