Sage Software of Rockville plans to merge with Index Technology of Cambridge, Mass., in a tax-free pooling of interests valued by one Index official at $52 million. After the merger, Kevin J. Burns, chief executive officer of Sage, will be chairman and chief executive of the new company, which will be renamed.

Signet Banking of Richmond delayed a decision on its fourth-quarter dividend to Dec. 28. The company pays a regular quarterly dividend of 39 cents.

Ryland Group is expected to name Roger Schipke chairman and chief executive of the Columbia, Md., home builder and financial services company, according to Dow Jones News Service. Schipke would replace Charles E. Peck, who is retiring.

General Dynamics won a $1.8 billion Air Force contract for 402 F-16 aircraft.

Merrill Lynch and Prudential-Bache will increase the commission rates they charge customers to buy or sell stocks. In another move reflecting the continuing slump on Wall Street, Pru-Bache and Shearson Lehman are discussing the possibility of combining their back-office operations, according to Wall Street executives.

AT&T will lower daytime long-distance telephone rates by an average of 1 percent effective Jan. 1, reflecting lower rates paid to local phone companies to begin and end calls.

USF&G's debt rating was downgraded by Standard & Poor's, in part because of uncertainties created by the recent appointment of a new chairman.