In Chapter 1, Mr. Shelby uses an idiom to describe Eliza's value to his wife after Haley suggests he sell her to him as well:
Mr. Haley, she is not to be sold, said Shelby. My wife would not part with her for her weight in gold.
The phrase "part with her for her weight in gold" is an idiom. To say something is worth its "weight in gold" means that it is so helpful it is invaluable. This use of idiom is extremely intentional; rather than having Shelby simply say no, Beecher Stowe cleverly inserts this idiom to reveal how Shelby, a "gentleman," views Eliza, a human being, as equal in value to a material object like gold.
Throughout Uncle Tom's Cabin, Beecher Stowe carefully uses figurative language to describe the impact the institution of slavery has on the novel's characters. In writing Uncle Tom's Cabin, Beecher Stowe wanted to underscore its horrors, highlighting how it corrupted the souls of those who participated in it. In Mr. Shelby's case, his use of idiom reveals something deeper about his character. Although he is depicted as kind and less brutal than other slaveholders, he ultimately chooses to maintain his family's financial security rather than protecting Tom and Harry, a choice that reveals how he is ultimately unwilling or unable to see his slaves as equals.