Verizon scored a bonanza last month when it sold a four-story commercial building in Harlem for $6.3 million -- a year after buying it for $190,000.

The sweetheart deal was part of a long-term lease agreement Verizon signed in the 1980s, when the cable and phone company agreed to rent the building at 148 W. 125th St., at Lenox Avenue, for 25 years.

A savvy negotiator included a clause that gave Verizon the option to buy the building at the end of its tenancy for $190,000.

Verizon confirmed it sold the property last month to Jackson Group LLC, a Manhattan-based commercial real-estate consortium.