CBS Inc. founder William S. Paley, chairman of the communications empire, has told friends he expects to leave his post at the end of the year, sources said today.

Paley, 81, has agreed to purchase a partnership interest in Whitcom Investment Corp., a firm that owns Whitney Communications Corp,. which in turn owns one-third of the International Herald Tribune, published in Paris.

Whitcom is a private concern and none of the transactions is directly related to Paley's CBS holdings, CBS officials and others said.

Paley, a legendary figure in the broadcasting business, founded CBS in 1928 and has been chairman of CBS since 1946. According to the CBS Inc. proxy statement, Paley owns 1.93 million shares of CBS Inc. or 6.9 percent of the company. Those holdings alone are worth almost $80 million.

The proxy statement also notes that if Paley ever retires, he will receive half his annual $339,746 salary until 1987.

Officials at CBS were caught by surprise by the news, first disclosed in New York Magazine by columnist Jack Egan. CBS spokesman said they had no knowledge of Paley's plans and Paley was headed for a vacation in Africa yesterday.

CBS Corporate Affairs Vice President William Lilley III said the company could not "confirm or deny" Paley's reported departure. He emphasized, however, that the company "is in no way involved in any negotiations with Whitcom."

It is also unclear precisely what Paley's move to the Whitney organization will mean for the Herald Tribune, owned jointly by Whitney, The Washington Post Co. and The New York Times Co. The paper was once owned by John Hay Whitney, Paley's late brother-in-law.

A Washington Post Co. spokesman had no comment on the Paley plan. But Arthur Ochs Sulzberger, chairman of The New York Times Co., said he expects Paley to become a Herald Tribune board member. Sulzberger added that at the Herald Tribune there would be "no structural change whatsoever in ownership, management or anything else."

Securities analysts and other CBS observers have long suggested that Paley's presence in the chairman's slot had made it difficult to move the firm swiftly into new ventures.

Some analysts speculate that one reason for Paley leaving CBS is that the Whitney firm owns cable television systems and federal cross-ownership rules generally bar broadcasters from owning both broadcast and cable properties.