The debt-ridden United Nations disclosed an unprecedented proposal to borrow $50 million on the open market to stay afloat. The idea came from Secretary General Javier Perez de Cuellar, who did not say what the world body would use as collateral. The U.N.'s most visible asset is its headquarters on 12 acres of prime real estate in Manhattan. In a written report to the General Assembly, which would have to authorize the borrowing, Perez de Cuellar said that all U.N. reserves have been exhausted, and there is a prospect of entering 1988 with barely enough cash in hand to cover two weeks' commitments. A principal reason for the financial dilemma is the withholding of funding by Washington, which has called for more controls over the U.N. budget.