Warner Amex Cable Communications said it has reached a preliminary agreement to sell its Dallas area systems to Heritage Communications for $110 million if necessary approvals are won from the cities of Dallas, Mesquite and Farmers Branch. Warner Amex has received a $1.25-billion bid for all of its assets from Time Inc. and Tele-Communications Inc., but Warner Amex spokesman Burt Solomon said the Dallas sale is not expected to unravel any bids. "Under any scenario for the sale of Warner Amex, the sale of Dallas to Heritage would go forward. All of the potential buyers are aware of the transaction," Solomon said. In interviews last year, Warner Amex officials said they had spent more than $150 million constructing the Dallas system.