If you have a valid contract to buy the property at the agreed price then you would seem not to have a leg to stand on. Look at it from the develper's point of view -- if the property had increased greatly in the interim, how would you feel if he tried to wriggle out of it?
A contract is just that, it locks both parties into an agreement to buy/sell at an agreed price. Your best hope is that he has inadvertently stepped outside the terms of the contract, i.e. unreasonable delay beyond the agreed closing date etc. You need to engage a lawyer asap.
In the event that the lawyer advises you as per above, your best bet is to negotiate with the seller. They won't really want to go to court and may agree to settle at a slight discount.