setting a fox to guard the henhouse; trusting a cat with milk
A rare proverb meaning to put someone in a situation where their natural instincts will lead to trouble, similar to 'setting a wolf to mind the sheep'. The literal image is giving a fox its favorite food, adzuki-bean rice, which it will inevitably eat.
See also: 猫に鰹節
This situation is exactly what they mean by 'setting a fox to guard the henhouse'.
A more common proverb with a similar meaning: putting a cat in charge of dried bonito flakes, which it will eat. 狐に小豆飯 is rarer and uses a fox and adzuki-bean rice.
A traditional Japanese proverb based on the idea that a fox loves adzuki-bean rice and cannot be trusted with it. The exact origin is uncertain.