If you think that they're dishonest with you, then ... play that dishonest game with them: you can tell them that you accept that onsite job and will be in the

If you think that they're dishonest with you, then ... play that dishonest game with them: you can tell them that you accept that onsite job and will be in the office at that time on that day. On the day that you're supposed to be in the office, you tell them that their office is too far and you cannot go there. They'll let you go and will also have to spend all the time and effort to find another person for your position. After a few times like that, they'll realize that it's better to be frank at the beginning.