After staggering the opening of rental units at 20 Exchange Place, developer Metro Loft Management has put the building’s remaining 200 units up for rent. The apartments are located on floors 16 through 57, and are available for immediate occupancy. Rents start at $2,100 per month, and prices are more than 15 percent less than the building’s rents were less than a year ago. To draw renters, security deposits will be waived for people with good credit. The landmark building’s leasing office opened in April.