If President Reagan sincerely believes it serves the cause of democracy to offer U.S. friendship to Nicaragua on the condition that it hold full and fair democratic elections within 60 days, then why doesn't he make the same offer to South Africa?

South Africa is a country far less democratic than Nicaragua; it is a racist state where three-quarters of the population are denied any vote solely because their skin is the "wrong" color. Yet Mr. Reagan remains "constructively engaged" with this tyrannical state, which denies democratic rights to the majority of its people. Therefore, can we, outside America, really be blamed for thinking that the U.S. president's only real interest in democracy is to use it as selective political blackmail in the game of international power politics? LEN CLARK Uxbridge, England

Would it be feasible for Congress to vote for further aid to the contras but to hold the monies until such time as the Nicaraguan government has proven its sincerity in abiding by the agreement made with the other Central America governments and the United States? If Nicaragua returned to a democratic form of government, the monies could then be used for the betterment of the health and economy of all of Nicaragua.

It might be worth a try to dangle such a carrot before a balky horse. KATHERINE J. KRIEGER Nicholasville, Ky.