FIRST FINANCIAL <FFMC> BIDS FOR COMDATA <CDN> First Financial Management corp said it has offered to acquire Comdata Network Inc for 18 dlrs per share in cash and stock, or a total of about 342.7 mln dlrs. The company said for each Comdata share it would exchange half a First Data share and enough cash to bring the total value up to 18 dlrs per share, provided that the market price of First Financial stock were not less than 28 dlrs per share. It said the cash payment would be based on the average market price of First Financial during a period shortly before closing. First Financial said it would not pursue the offer if Comdata's board rejected it. Comdata has already agreed to be acquired by a partnership for either 15 dlrs a share in cash or at least 10 dlrs in cash and uniuts of securities. The partnership that made the first offer for Comdata was Welsh, Carson, Anderson and Stowe. Comdata had previously entered into an agreement, which collapsed, for a repurchase of six mln shares at 14.50 dlrs each and for the sale of one mln shares by a director to <Mason Best Co>. Mason Best already owns 1,800,000 Comdata shares. A group led by <Rosewood Financial Inc> has also disclosed ownership of 6.2 pct of Comdata's 19.0 mln shares outstanding and said it might seek to increase its interest to over 15 pct. The company said Comdata shareholder approval would also be required for its proposal.