Karius spent last season on-loan with the Bundesliga outfit, but only managed five appearances.
"When I moved to Union, I was told I would play. Otherwise I would not have gone there," he told BILD.
"Everyone told me that I was incredibly important to the team and they are sad about how it has gone. When you look at it all, it was definitely not fair because of what they had told me."
He added: "I want to be in Liverpool on July 12th. There is still a lot going on at the moment.
"I want to keep everything open and make a decision."
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