It amazes me how many people are incapable of putting their position across rationally.
You're being asked to do another job and in order to do that job, they'd like you to be part of the team and move from your current area. This is perfectly understandable.
Your problem now becomes the fact that the position is apparently only temporary and also that you're going to be on the same pay, but with more work/responsibility.
Your options are:
- Politely decline the new position, stating that you are happy in your current job.
- Accept the new job outright and put up with it.
- Talk to your boss about the new job, pointing out your concerns about not being paid in line with others doing that job.
You have to decide if accepting this job is likely to benefit you and your career in the long run. It may be temporary now, but is it likely to lead to bigger and better things? There may not be a wage rise now, but will this be reviewed in the (near) future? Are you just being taken for a mug and asked to do something, because they know you're the sort of person who'll quietly moan, but ultimately put up with it?
Basically if this job is likely to in any way benefit your career, then I'd talk to your boss and try to figure out what the future is likely to hold in this new job.
But if it is just a 'job' and you see your future elsewhere, then just stick with your current role. No point accepting more stress, for the same pay if you've no plans to move up the ladder.