I think the ruling on avoiding to pay tax in the west by getting paid in cash is haram, this is what I read from one of your responses here. But what is actually haram the money gained or the act of avoiding the taxes, or both ?

Avoiding taxes is forbidden. This is the fatwa of Sheikh Mohammed Hisham, based on a conversation I had with him regarding our brothers in the West, as this is a recurring theme. What he mentioned is that because you are in their land, you have accepted their terms. Therefore, it is not allowed for you to use this type of trickery to violate what you agreed upon.