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The place where you air your problem, at least initially, is with the
agency that placed you. You and they have a legal agreement, supported
by your resume, about what you were being hired to do. If they and the
client screwed around with that, that's a problem between them and
you're only peripherally involved, at least at first.
The fact that the client is currently advertising for people with your
skillset can be offered by the agency as a way out of the situation -
with your permission, they could substitute you at one of those
positions and continue to look for someone with the qualifications to do
the hardware manual for which you don't feel qualified.
Many companies truly don't understand that there are varieties of
specialties within the technical writer apellation - they assume that if
you can write tech about one subject, you can do it about any subject.
Just how these companies get educated is another discussion.
Just as an aside, have you considered the possibility that, even though
you don't feel qualified to do what you've been asked, (1) the company
is desperate to get this particular job done, and (2) you have an
opportunity to learn something new? You can easily make the case that
part of a technical writer's job is to learn the new product/system, and
translate both the process of learning and the product knowledge itself
into useful material in a manual. Of course, this may not be possible
in that particular company, but it sometimes works.