The 39-year-old has been linked with a move away from the Coface Arena, with reports suggesting Tuchel will join the Gelsenkirchen side at the end of the season.
Jens Keller has been appointed interim coach at the Veltins Arena until the end of the current campaign, but Heidel says he is not interested in opening negotiations, regardless of the size of the bid.
“They can offer what they want,” Heidel told Bild.
"They can gladly increase their offer to 60 or 70 million. Even if we are approached by a Russian or some such Sheikh, I will also say the same, so you can end this topic!”
Mainz are currently sixth in the Bundesliga after taking 26 points from 17 games.