The Boston Teachers Union is requesting $83.5 million more than the city for salaries, a hefty financial divide that is slowing contract negotiations, according to a report being released today. The salary proposal pitched by the teachers union would cost the city $116.1 million over four years, while the city’s offer would cost just $32.6 million over the same period, according to the report prepared by the Boston Municipal Research Bureau, a fiscal watchdog funded by businesses and nonprofit institutions