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

Understand the thought

Meaning in plain English

Possessing a large amount of something does not help if it cannot meet the need. The cat among grain highlights the difference between abundance and usefulness, without assuming that money or other resources are always useless.

What the words are saying

The cat is surrounded by a plentiful resource that does not provide the benefit wanted in the image. The saying applies that mismatch to someone whose apparent riches are not available for the purpose at hand. A person or group may have substantial materials, commitments or nominal funds while lacking what the immediate task actually requires. The phrase does not establish why the cash cannot be used, so no particular restriction or financial arrangement should be inferred. The useful question distinguishes what is possessed from what is accessible and suitable, rather than treating an impressive total as evidence that the need has already been met.

Everyday example

A club has many donated display boards but still cannot hold an outdoor event because those boards are unsuitable for the available space and it lacks permission to use another site.

When to use it

Use it when visible abundance is being mistaken for resources suitable and available to the task.

Agent-assisted editorial interpretation. This explains the words; it does not by itself verify their author or earliest origin.