Tuchel had announced he will leave at the end of the season, but fans are petitioning him to stay.
The German has been linked with West Ham and Manchester United and he said: "There is an agreement so it cannot be changed.
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"But you can change all contracts by mutual agreement because we had a contract but both parties agreed to break the contract early, so anything is possible.
"But the answer is the same, the agreement exists and it stands firm."