A former police officer confessed to slaying Argentine businessman Osvaldo Sivak one day after Sivak's family paid a $1.1-million ransom in 1985, court sources in Buenos Aires said. They said that Jose Lorea, a former police inspector arrested with three other former policemen last week, told a federal judge that he killed Sivak on Aug. 12, 1985. Sivak, a real estate executive, was kidnaped near his Buenos Aires home two weeks before. His body was found by police last week in a field 35 miles south of the capital. The case sparked controversy in Argentina after early investigations pointed to involvement by right-wing death squads.