Hi Mana,
The first thing you have to ask before even considering the savings options is if the property is really worth what you have negotiated. If i were you i would instruct an independent valuer to have a look, it may be that the property hasn't infact been undervalued, but infact you agreed to pay too much for the property in the first place. From the amounts mentioned, it seems like this is a lot more than a couple of grand!
If you do decide to pay the shortfall direct to the builder, in a couple of years time if it's found the property was worth less than what you paid, you could have problems getting a new mortgage due to a potential negative equity situation.
What you should try to remember is that at the moment it is a buyer's market, and builders especially are taking a hit. I would ask for the full reduction or pull out.
Hope things turn out well for you.
Michael