The NRW Film and Media Foundation, Germany's largest regional film funding body, later retracted the statement, first reported by The Hollywood Reporter.
The NRW statement was part of an announcement of new funded projects. NRW said in the retracted statement it would invest about $1 million to co-finance the film, the Reporter said.
The film is to be shot this year and open next year.
CAA, Hollywood's leading talent agency, said Firth and Blunt -- who star together in "Arthur Newman," opening in the United States Wednesday -- are not involved with "Murder Mystery," the Reporter said.
Sandler's representatives had no immediate comment. Theron -- who appeared in "Snow White & the Huntsman" and "Prometheus" -- is one of the "Murder Mystery" producers as well as an actress in it.
The story by "White House Down" screenwriter James Vanderbilt involves an American couple in Europe on their honeymoon who witness a murder, are wrongly accused of committing it and then go on the run.