Banker & Tradesman file photo

The downtown Boston office market was in ascent in 2016 when a New York developer paid $27.5 million for a 70,000-square-foot Leather District building.

The price was nearly twice the property’s previous trade in 2013, following double-digit rent increases for class B office space.

This week, City Realty Group took advantage of the steep discounts available in the office market to acquire the 6-story building from MCRE Partners for $13 million.

207 South Street Financial LLC, a Boston corporation that lists William Thibeault as manager, provided $9.5 million in mortgage financing for this week’s transaction.

Class B properties have comprised the bulk of Boston office investment sales in the past year, after steep declines in occupancy and rent.

The class B vacancy rate in Boston is currently over 21 percent, according to a report issued this month by brokerage Hunneman, compared to 12.1 percent at class A properties.

The South Street property also traded for $14 million in 2013, when Meritage Properties of Scarsdale, New York acquired it and launched a renovation program.

City Realty Group acquired another Leather District office building at 186 Lincoln St. last fall for $11 million, announcing its plans to acquire and stabilize downtown properties.

In May, the Allston-based firm bought the Chestnut Hill Office Park in Brookline for $41 million and announced mixed-use redevelopment plans.