Rolling in cash he can't use, like a cat in a corn-bin.

A person may have plenty of something and still gain no benefit from it. The cat is surrounded by corn, but corn does not meet its needs.

Here, the comparison makes unused wealth seem absurd: having a great deal of money matters little if the person cannot put it to any useful purpose.

Use it to point out the mismatch between someone’s apparent riches and what those riches can actually do for them. The image gives the observation a wry, concrete edge rather than stating it as a general rule.