"Finally, we note that even if Mr. Johnson had succeeded in establishing the authenticity of the documents he submitted, Mr. Baker would still be entitled to prevail. It is a well-established tenet of contract law that once a party has formed a contract by accepting an offer, he cannot subsequently vary the terms of that contract by presenting a counter-offer to the other party. Both versions of the Purchase and Sale Agreement contain Mr. Johnson’s signed acknowledgment that the buyer’s offer was accepted at 2:00 p.m. on July 26, 2005. The signature on the purported counter-offer bears the same date, but recites a time of 4:30 p.m. Thus, even if such a counter-offer had been presented to Mr. Baker, it would have had no legal effect." Id.