Not entirely sure what exactly you're asking. If the builder finds that the first of the two houses sells off the plans easily, s/he might decide to ask €310,000 or more for the second.
Or are you asking if you can haggle with a builder over the price? Yes, although in the current market I doubt that you'd get very far.
Either way, until contracts have been exchanged, the price of any property is open to negotiation, up or down.
