Seller/builder backing out of an agreement- advice requested

[Kent Baltare]

Here is the scenario,

Signed a purchase contract 4 months ago to buy a home that was being constructed.  Purchase price is $X and I gave a 10% down payment, as per our agreement.  Since then there has been an uptick in the market and the completed home could now be sold for about X+ 20%. 

Now in in the homestretch with closing, I am starting to get some push back from the seller indicating that he may be trying to tank the sale, so he can relist and capture that appreciation.  He could do this by extending closing until I bail out or not completing the place to spec.

I am wondering if anyone has been through something like this and if they have any advice on what options there are. 

PS the place is in TN.

Thanks for your time.