Read the contract. Is there some provision for cancelling the contract if it fails to appraise for the stated amount? Is there something in the provisions about financing (since you may not be able to get financing unless you ante up the difference between sales price and appraisal)? Unfortunately, contracts written by builders are often very one-sided.
Check with the Realtor who represented you. (You did have one, didn't you?) He/she can't give you legal advice, but may be able to suggest a strategy, such as negotiation, especially if you're still interested in the condo.
And that could lead you to a real estate lawyer. You did have one review the contract, didn't you?