President Reagan's plan to nominate Robert J. Brown, a black North Carolina businessman, as ambassador to South Africa ran into what administration sources described yesterday as "a hitch." The sources added that Brown's name had been removed from the draft of a speech that Reagan intends to make Tuesday about his South Africa policy.

The nature of the "hitch" -- and whether it means that Brown's proposed nomination might not go forward -- was not immediately clear. It appeared to be related to rumors circulating yesterday about whether Brown had been involved in questionable deals to sell rice to Nigeria.

In a telephone interview yesterday, Brown denied that he had ever been involved in Nigerian rice sales. Later last night, Brown added that the reports of a hitch were "news to me," adding that no one in the administration had contacted him about the rice sale rumors.

The rumors, while vague, centered on Brown's relationship with Umaru Dikko, a former Nigerian cabinet minister who fled to Britain after a military coup on the last day of 1983. Dikko has been described as "the most wanted man" by Nigeria's current military rulers, who accused him of massive corruption and the theft of millions of dollars.

In a sensational incident two years ago, Dikko was kidnaped from his London home, allegedly by former Israeli commandos working for Nigeria. The plot was thwarted when British authorities found Dikko drugged and stuffed into a crate waiting to be loaded aboard a jet bound for Nigeria. The crate, marked as "diplomatic baggage," was addressed to the External Affairs Ministry in Lagos, the Nigerian capital.

In the interview from his public relations firm in High Point, N.C., Brown said he has been a close friend of Dikko since 1979 and, at Dikko's behest, had acted as an adviser on election strategy and public administration to the government of deposed president Shehu Shagari, Dikko's brother-in-law.

"Umaru Dikko is my friend," Brown said. "He was then. He is now. He will be in the future, whether I am an ambassador or a private citizen. He is a tremendous man -- a great man who got a bum rap and who has been vilified because he spoke out against the military who overthrew democracy in his country.

"My work for Nigeria was exclusively in political consulting," Brown added. "I never had anything to do with rice sales or any other sales. You can look at every document in Nigeria or in this country involving their purchases of rice, and you will find that my name has never appeared as an agent for any rice dealer."

The questions about Brown's links with Nigeria followed revelations that in 1973, after serving as a $36,000-a-year White House aide to then-president Richard M. Nixon, Brown was certified by the Small Business Administration as "financially and socially disadvantaged" and was given an $860,000 contract to provide food and janitorial services to a California military base. Although the job was reserved for a firm owned by a minority-group member, an SBA probe found the contract actually was administered from behind the scenes by a white businessman.

However, administration sources said, as late as yesterday afternoon, Reagan reportedly told aides that he thought nominating Brown as ambassador was a good idea because it would help to improve the U.S. dialogue with South Africa's black leaders. It was not until later in the day, the sources said, that Brown's name was taken out of the speech that Reagan is scheduled to make at 2 p.m. Tuesday in the White House East Room before a joint meeting of the World Affairs Council and the Foreign Policy Association.

While noting that he had not been offered the ambassadorship formally, Brown said in yesterday's interview, "I've had to pray over this a great deal, and I believe I've received a message that this is something the Lord wants me to do."

He said that after he had prayed for "a sign" on Thursday night, his first phone call yesterday was from a woman in New York who told him that black friends in South Africa had asked her to advise him: "Everyone is praying God will send you there. They are waiting for you."

"I knew that God was sending me a message," Brown said. "That's where my life is if the president asks me to go. When I meet my maker, I know that when judgment is passed on me, it will be said that I did my duty to help my country and my black brothers."

Brown said that he first met Dikko on a 1979 trip to Nigeria, the largest country of black Africa, after Shagari had become president in a disputed election. He said their talks developed into a close relationship where Brown counseled the Shagari government on administration and training programs for bureaucrats and legislators. He even arranged for a black American artist to design a presidential seal for Shagari.

In 1983, Shagari ran for reelection in a campaign managed by Dikko. Brown said he advised the Nigerian campaign staff on advertising and television techniques. Although Shagari won the election, opponents charged him with vote fraud and with financing his campaign by extorting bribes from people doing business with the government. On New Year's Eve that year, the Nigerian army, citing "intolerable corruption," ousted Shagari and made Maj. Gen. Mohammed Buhari head of state.

Several American diplomats, business leaders and journalists familiar with Nigeria said yesterday they were aware that there had been a close relationship between Brown and Dikko. But these sources, who asked not to be identified, stressed that while Dikko was known as "the king of bash" -- the Nigerian term for bribery -- they had no knowledge of Brown being involved in anything illegal.

Some of the sources said they had heard rumors that when Dikko visited this country in 1983, Brown had acted as a middleman in arranging some business deals. Brown said yesterday that Dikko was here to receive some honorary degrees and that his only service for him on that trip was to arrange a meeting with Vice President Bush.

Staff writers Lou Cannon, David Hoffman and Don Oberdorfer contributed to this report.