Are verbal negotiations binding?

The seller didn't accept my initial offer to buy his home. He told my agent he would sell the house to me if I came back with an offer $10,000 higher. So I did ... only to find out that the seller is already under contract with another buyer. Didn't he have a duty to sell to me if I met the asking price he said he would accept? No. Although buyers and sellers may have verbal communications to negotiate acceptable terms, a written contract must be signed by both parties to be binding.