This should not be allowed. If a requester posts a job for a price, then they should be held to the price upon completion of the work. I almost feel silly saying anything considering that some requesters will just take your work and reject for no reason, but in a way this seems even worse to me. If certain requesters caught on to this I could easily see them abusing this worse than those who flat out reject. A lot of workers don't want to do HITs for unreasonably low pay, so they post at a reasonable rate and pay at a crap rate.