Has anyone in contemporary ethics discussed whether or not there is a difference between (1) taking something away (e.g., stealing $100) or withholding something (e.g., refusing to pay $100 of taxes)?
We can easily trace this idea back to Marx. 'Exploitation' in the Marxist sense is the capitalist's refusal to give laborers a value commensurate with their actual labor; for Marx, that is the theft of labor value. You'll find versions of it running through most social theory and philosophy that engages Marxist thought (which is, frankly, most social theory and philosophy). People don't often say it outright — because the kind of people they would accuse of theft in this context are extremely rich and powerful, and those accusations have unpleasant consequences — but it's hard to avoid the idea, even if we do something as mundane as listen to Elizabeth Warren or Bernie Sanders.
When concerning ethics both are the same. But most people think is that the latter as less offensive. I think most people don't think much about its roots.
Usually, before or after purchasing a commodity we pay the bill amount. If you refuse to pay the amount after purchasing a commodity, it is cheating (in broad daylight). Since it is in broad daylight we never call (we can't call) it stealing. Instead, we call it cheating. The change is only in the terms.
From your example we are likely to consider withholding of tax only. But we should consider all the similar things that are related to withholding. (Your 'taking away' also can be done in broad daylight. We should not forget it.)
In 'taking away', somebody's effort has already been transformed as money/something. But in 'withholding', often, the effort has yet to be transformed as money/something.
When you steal something, often the victim doesn't know the thief. But in the case of withholding, the victim knows the thief.
So, if your thought is in the right path you will realize that practically/ETHICALLY also, the latter one can't be placed in a higher position.
Some people might say that taking away of money or things that is made illegally and refusal of paying tax are ethical. Though illegal, everybody has no right to seize it from the wrongdoer. If so, the person's (who seizes) act also becomes unethical. We can decide the ethics of such cases only by assessing the doer and purpose of that action. Some times we could treat 'the counter attack for saving the downtrodden, nation etc when all other possible routes are blocked', as ethical. 'Salt satyagraha' that was conducted during the freedom struggle in India was one such act.
(This exceptional case is not a comparison between the two. To get a clear picture about ethics, knowledge about its 'capillaries' also is worth mentioning here.)