Trade talks between the U.S. and China have hit a snag over farm purchases, as officials seek to lock down the limited trade deal President Trump outlined last month.

Mr. Trump has said that China has agreed to buy up to $50 billion of soybeans, pork and other agricultural products from the U.S. annually. But China is leery of putting a numerical commitment in the text of an agreement, according to people familiar with the matter.

Beijing...