In Chapter 43, Isabel discovers that her mistress had not actually sold Ruth but that she was in Charleston in the Lockston's estate. She discovers this fact in the most cruel way possible, as her mistress deliberately punishes her by threatening the life of Ruth. Note what she says to Isabel and the way in which she tells Isabel about Ruth's fate:

Couldn't find a buyer. Had to ship her down to Charleston. I shall tell the estate manager to get rid of her, toss her in the swamp. Her death will be on your head, you insolent fool.

Isabel thus discovers that Ruth is not actually lost and she resolves to do everything she can to rescue her and bring her to safety. Although the novel ends before the reader discovers whether Isabel is successful in her plan to save Ruth, it is clear that Isabel's plan having escaped is to walk to Charleston from across the river and try and find Ruth there in order to rescue her. The precise fate of Ruth therefore is left open in the novel. The reader is only told that she is in Charleston and not sold to some unknown buyer, therefore giving hope that Isabel might be able to rescue her.

What happens to Ruth at the end of Chains?

Unfortunately, readers are not given this information. Earlier, readers and Isabel assumed that Madam Lockton sold Ruth and that the likelihood of Isabel finding her is very unlikely. Then in chapter 43, Isabel bravely defies Madam Lockton's demand to hand over a note. Isabel throws the note into the fire, and Madam Lockton is so angered that she threatens to sell Isabel and Ruth. Isabel and readers are caught by surprise because we learn that Madam Lockton didn't sell Ruth.

"Couldn't find a buyer. Had to ship her down to Charleston. I shall tell the estate manager to get rid of her, toss her in the swamp."

It was a lie to intentionally upset Isabel. Madam Lockton couldn't find anybody that wanted to buy Ruth presumably because of her mental disorder. This information propels Isabel into escaping the Lockton household by forging her freedom papers and escaping New York. The book ends by giving readers the impression that Isabel will seek out and find Ruth. Book two in the series is focused on Curzon. He and Isabel did not go together to find Ruth, and readers will have to continue on to book three in the series for further information about what happened to Ruth after Madam Lockton sent her away.