یک ضربالمصل: 

به گربه گفتند فلانت درمانه، خاک داد روش

Here’s an idiom for you: 

They told the cat “your poop has medical properties,” he covered it with dirt

See, the cat’s there, the poop of no value to him, but the second you point out that you might want it, he quickly makes it unavailable.  This idiom applies to situations when you need a small piece of info from someone, and they suddenly make it difficult for you. 

It’s also known as “rent-seeking behavior”