Seller doesn't want to extend closing date

[Richard Aaron]

Hello Bigger pockets members. I have a contract issue that I could use some advice from some experienced investors or attorneys out there. 

Synopsis: Seller and I signed a contract  on July 3rd 2018.   Closing date was supposed to be August 31. The loan is with the Veterans land board. In Texas, and they take a long time.  The property finally got approved by the VLB on August 30. Problem is the Contract expired August 31.  I sent an amendment to the seller August 30, to extend the closing date another two weeks which is what the VLB needs. The seller did not sign the amendment. In fact the seller texted me on at least two different occasions that he was feeling very impatient and that he had two other cash offers waiting in the wings.  Normally I wouldn't believe this stuff but in this case I think it might be true because I got such a good buy on the property.  So I'm thinking he's going to claim the contract date of August 31,  as the end of the contract  and go on to his cash buyers. 

I really want the property and the protracted time is through no fault of my own.  Here we are down to the final two weeks, I am approved and the property is approved and the seller wants to back out. 

 So I started combing through the contract. As I combed through the contract I noticed on paragraph 22 that it states that "This contract contains the entire agreement of the parties and cannot be changed except by their written agreement."

Three days after we signed the contract the title company told us we needed to sign an HOA addendum.

We both signed it without a proper TREC approved amendment, and there's no date on the HOA addendum.

The issue I have is in paragraph 22 where it says "This is the entire agreement....". well it seems to me like it wasn't  because 3 days after the contract date this addendum got added arbitrarily.  Additionally the box for adding that addendum under paragraph 22 on the contract is not checked. I understand it was necessary for the seller to cover himself but still it's an arbitrary change to the contract. 

I said all the above to say this: Since the seller arbitrarily changed the contract  could he still insist on a contract termination date of August 31? Could I sue for specific performance?   Or maybe threaten to sue for that or breach of contract, or something else?  Or should I just go on to another deal? 

Thanks