I.M. Pei's New York home open to offers

That first meeting led to Cheng's decade-and-a-half involvement with the Peis, in which he helped with financial and legal issues and frequently joined them for breakfast and casual chats.

One time, when Cheng visited the couple after he had been away for 10 days, Pei greeted him with "Long time, no see."

"I said, 'No, Mr. Pei, only 10 days.' He said, 'No, Kai, you don't understand. For people my age, 10 days is a very long time.'"

Many of their conversations took place in the library on the second floor, next to the spacious, bright living room that hosted prestigious guests, including cellist Yo-Yo Ma, violinist and conductor Itzhak Perlman and former US secretary of state Henry Kissinger.

The small library, off-limits to guests, was one of Pei's favorite places in the house. His discussions with Cheng, shared over glasses of whiskey, ranged from architecture to business, investment, politics, friendships and raising children.

When asked about his favorite project, Cheng said that Pei drew a parallel with his family, saying that he would not answer who his preferred child was.

"He always had a funny way of countering," said Cheng, who witnessed several playful exchanges between Pei and his wife, who were married for more than 70 years.

"One time, I remember Mrs. Pei being funny. She said to her husband: 'You are too old. You don't remember my name now'."

Cheng said Pei replied, "Well, I may not remember your name, but I remember I love you."

The last photo of the couple, taken in the library in April 2014, has been kept by Cheng.

The room remains unchanged, with the brown leather sofa in front of four shelves of books stretching to the ceiling.

"The library is less-known by the world, but very important for the family and myself," Cheng said.

Joseph, the Christie's broker, said that since the house had been put on the market, it had attracted interest from everywhere, including people living locally and overseas who "are big fans of Mr. Pei and want to be a part of this legacy".

Cheng said that some prospective buyers want to keep the house as it is, and would like to invite the Pei family to visit whenever they wish.

While the sale proceeds, the family is trying to ensure Pei's legacy is respected, and is discussing ways to ensure this, including donating some his wife's clothes to museums to "preserve her love for fashion", he added.

A walk-in closet at the house, full of her shoes, stands as evidence of her passion for style.

The residence is one of two owned by the family in New York state, the other being a summer house 64 kilometers north of the city in Katonah.

Pei had visited many different cities worldwide, but he told Cheng that there was no place like New York.