A South American family has paid close to $27 million for eight units at Waterline Square, a new construction residential complex on Manhattan’s Upper West Side, according to the buyers’ agent, Maria Velazquez of Douglas Elliman.

The buyers went into contract and closed on the all-cash deal on the same day, an extremely fast turnaround for the New York market, said Ms. Velazquez. She said her clients received a blended discount of more than 7% from the asking price of the units, which range in size from one to three bedrooms...