Are you asking whether the client can change the price? Some clients put any number into the project rate because they're not sure yet how long the project will take, what the freelancer will bid, etc. They will then negotiate BEFORE sending the official job offer (it doesn't make much sense to do it afterward). If the freelancer likes the terms, she'll accept the job.
If you're asking whether the freelancer can change the price, a freelancer might bid a different rate than is listed in the project. We have many reasons for this. Sometimes it's because we realize the client only put in an amount because the space needed to be filled out, and the client is waiting for us to put in our own proposed rate. Once again, though, the freelancer can't change the rate (or rather, ask for a changed rate) AFTER the job is accepted, usually. That just isn't professional or in good form. If it turns out the job is very different from expected, or if it is fixed-rate but will take much longer than expected, bonuses or more milestones can be created.
