re;above
You cannot walk away, you signed the contract and it is a legally binding agreement.
I'd first go and get approval again, you may not (wont) get as much as 2007. Was your 2007 approval for much more than than purchase price?
I'd then get the valuation done for the bank, see what price they put on it.
In a lot of cases recently people who bought off the plans have been having trouble with new approval and new valuations.
If you do get both sorted ok, i would ask the bank for a letter saying you only have approval for purchase price - 10%. Take it to the developer and say this is all you can get and you dont have any more money.
You can try your best to get a discount, but you agreed to the price in 2007 and the developer is under no obligation to give anything off.