Is it common for professors to ask a postdoc to start working without compensation while waiting for paperwork to be completed? I was wondering if it is common for professors to ask a new international postdoc to work without compensation while waiting for paperwork to be completed.
I was in an interview and the professor hinted at something along this line, when I brought up the topic of the lengthy visa process to be expected.
A few notes:

The professor is not my PhD advisor. He's at a different university in a different country. So, my question is not similar to this one.
The whole visa process could take 3 months.
The work contract may include a start date that is earlier than the visa issuing date (in this case ≈ 3 months earlier) but the university cannot pay me for that period without a valid document/visa. That's why I mentioned 'without compensation'. Correct me if you think I am misunderstanding this aspect.
I understand that the scope of the postdoc's work is related to a project that has tight deadlines, which is why the professor wants work done as soon as possible. But, I find that the duration in which this situation would last is long. A maximum of 1 month may seem tolerable but not longer.

Is this common in academia for international hires? And would it be ethical?
Asking someone to work without compensation is illegal in many jurisdictions. It is also hardly ethical. Unfortunately, it is also not uncommon in academia.
In modern &quot;publish or perish&quot; academia professors are often under a huge deal of pressure from university administration to produce countless high-quality papers and teach ever-growing number of courses. A significant proportion of academics feel that they are forced to work extra hours to meet the expectations of their universities. This draining and insecure work environment often impacts the judgement of academics when they manage the work of their PhD students and postdocs.  Many  sincerely believe that they should &quot;prepare&quot; their students/postdocs to the hard realities of academia and make them more competitive by demanding to produce results at a cost of personal life and well-being. This is wrong, but unfortunately, wrong things do happen in this world and in academia as well.
UPD: Answering some questions in comments, I feel that I have to explain why the PI's behaviour is wrong. I don't think it is a subjective judgement.

Forcing someone to work without compensation and without contract is modern slavery. Slavery is wrong. Postdoc are employees, they have terminal education degrees, they are professionals and they do important research work, not some glorified &quot;training&quot;. Forcing a postdoc to work without contract and compensation is wrong.
But perhaps, the PI was not forcing the postdoc, but merely asked them nicely whether they would like to volunteer? Well, let's see. Postdoc does not know whether they can say &quot;no&quot; to their PI without risking their contract or compromising the work relations with their PI during their contract. There are so many ways in which postdoc's whole career depends on their PI's opinion (access to research resources, appraisals, extensions of contract, letters of recommendation, etc).  In this situation postdocs are under a huge pressure to say &quot;yes&quot; to unfair and illegal proposals. That's why even proposing to work without contract is abusive, morally wrong and often illegal.