The United States has paid $28.8 million in back dues, but a U.N. spokesman says the American government still owes the United Nations $570 million. The United States has withheld payments to try to force the United Nations to adopt budgetary and administrative reforms. President Reagan announced Sept. 13 he was satisfied with progress on the reforms and approved the release of $188 million - $44 million for the last fiscal year and $144 million for the one beginning Oct. 1. Two other payments have been made since Reagan's announcement - $15.2 million in September and $85.6 million Oct. 14. That leaves $58.4 million still outstanding of the amount Washington has promised, and more than $500 million more in back payments.