Children’s chapter book from 70s or 80s - rich recluse designs a game for his city, solved by boy

[Berta]

A mysterious rich man sets up a contest for the city he lives in. A school boy follows the clues and solves the mystery and ends up finding the man, dead in a tunnel in his backyard, where he had been burning his money to keep warm. (Not “The Westing Game” - but sort of similar idea.)

[Margie]

Hi Berta,
It sounds like it could be The Treasure Trap by Virginia Masterman-Smith, which was later released with the title The Haunted Mansion Mystery after it was made into a TV movie starring a young Christian Slater (https://www.imdb.com/title/tt0085651/). SPOILERS: The two kids searching for a million-dollar fortune follow clues to find a hidden tunnel and room underneath one of their houses. They discover that the man had burned his money while in hiding and then died. The man hadn't set up a "contest" but rather disappeared with everyone in town wondering where his fortune had disappeared to.