The chief Libyan diplomat in Washington lent credence last year to Billy Carter's claims that he could successfully broker a deal for more Libyan oil for a Florida-based firm.

In a formal statement last night, the Charter Co. of Jacksonville said that its contacts with Carter and the Libyans in discussing the proposal were "above-board at all times" and without any hints of impropriety.

Emphasizing that its discussions with the president's brother were "not initiated by us," the Florida-based conglomerate issued a five-page statement on the matter in apparent hopes of making it the last word on the issue.

In forcing Billy Carter to register as a foreign agent for the Libyan government last week, the Justice Department charged that Libyan officials had held him out to the U.S. business community as a commercial intermediary through whom U.S. business entities could deal in Libya.

Carter has acknowledged dickering with Libyan officials since the spring of 1979 about getting increased oil allocations for a Charter Co. subsidiary, but he has also denied having any influence with the Libyan government in commercial matters.

Outlining its dealing with the president's brother, the Charter Co. pointed out that Billy Carter suggested to one of its executives, Lewis Nasife, at an Aug. 17, 1979, meeting that he could obtain up to 100,000 barrels per day of Libyan oil for its subsidiary, Charter Crude Oil Co.

Stating that it sought to determine how real the prospects were, the Charter Co. said it was aware at the time that Billy Carter had hosted a Libyan trade delegation to Georgia "and appeared to have a line of communications with at least some Libyan officials."

In addition, Nasife contacted Ali Houderi, the chief Libyan diplomat in Washington.

"Mr. Houderi indicated that Billy Carter had a good rapport with his Libyan friends and that there may be a potential for his obtaining Libyan oil," the company said. "Based on this information, Charter determined that the Billy Carter contact had some merit.

Nasife tentatively agreed to pay Carter a commission of 5 to 55 cents for each additional barrel of oil he could obtain, and continued dealing with him through April of this year, the company said. By then, Carter had lowered his slights to obtaining 50,000 to 60,000 additional barrels a day. The company said it remained interested at its last meeting with Billy Carter on April 1, but that occasional telephone calls after that produced no progress.

"At no time did Billy Carter ever represent himself as anything other than a businessmen with a potential business venture," the statement said. "The contact was above board at all times, and no money for any reason exchanged hands. There was no attempt or suggestion to hide any payments if the deal were consummated."