The 1.2% drop was blamed on a slight increase in mortgage interest rates. The seasonally adjusted annual rate of home sales dipped to 3.41 million units, compared to 3.45 million units in May. The slower sales pace was accompanied by a $900 decrease--to $82,300--in the median price of an existing home between May and June, the National Assn. of Realtors said. Association economists said the drop was a market correction following the $3,000 jump between April and May to $83,200. The June median price was 7.6% higher than the $76,500 median recorded in June, 1985.