Chinese President Xi Jinping shakes hands with President Donald Trump before a bilateral meeting during the G20 Summit on June 29, 2019 in Osaka, Japan.

A leading commodities expert at Goldman Sachs has raised doubts over China buying at least $40 billion worth of U.S. farming goods to satisfy terms of the "phase one" trade deal.

The U.S. and China plan to sign the first draft of the trade pact on January 15 which will see tariffs eased and changes introduced to rules surrounding intellectual property and technology. Washington is also demanding that China buys between $40 billion and $50 billion worth of agricultural goods from the U.S. each year.

Speaking to CNBC's "Street Signs" on Friday, Goldman's Global Head of Commodities Research Jeff Currie said U.S. live stock and agricultural prices couldn't yet factor in a coming boost to demand.

"There is still a lot of uncertainty about how you would achieve $40 (billion) or potentially even $50 billion of agricultural purchases," Currie said, before adding, "A lot of the people I talk to are really skeptical that you can really achieve that number."

Annual U.S. farm exports to China typically exceeded $20 billion between 2010 and 2018, before falling sharply to around $13 billion over the last two years as trade war duties impacted sales of soy beans and pork.

U.S. Trade Representative Robert Lighthizer told reporters in December that China has pledged to buy a total of $40 billion in agricultural products, although President Donald Trump is calling for $50 billion.

The $40 billion figure represents a $24 billion a year baseline based on 2017 figures, plus an additional $16 billion for at least two years.