temptation one can't resist; setting a wolf to mind the sheep
Idiomatic expression likening a situation to leaving dried bonito flakes in front of a cat—something that will inevitably lead to trouble or temptation. Often used to describe a person being placed in a situation where they are bound to succumb.
Leaving money with him is like setting a wolf to mind the sheep.
Putting a cake in front of her when she loves sweets is like trusting a cat with milk.