The advert gives an indication of how much the employer is prepared to pay for the role. The amount the contract states is how much the employer (after interviews etc with you) is prepared to bid for your services. This is a bit like an auction in that the employer wants to pay as little as he can, and doesn't want to show his hand on how much he has in the pot (He's shown a bit by the advert, but is that the absolute value?).
You can, of course, reply and say that the bid is too low, using the advert as leverage. The employer is within their rights however to say the original amount is the final offer (or even that the deal is no longer available), that is the prerogative of both you and the employer, to change or back out the deal before it is completed. The trick is to get the best you can, whilst keeping the employer interested.
If you accept, then re-negotiate, you are then potentially breaking a contract, and you will risk losing it all.
Think about what you truly would be looking for, and can justify. Counter that to the employer, but be prepared for them to walk away, esp if a much higher number than the offer.