Why is Eliza separated from Emily when she's sold to the Fords?

Freeman refuses to sell Emily with her mother because he plans to keep her until she is older, when the beautiful, young slave will be worth a huge amount of money.

He would not sell her then on any account whatever. There were heaps and piles of money to bemade of her, he said, when she was a few years older. There were men enough in New-Orleans who would give five thousand dollars for such an extra, handsome, fancy piece as Emily would be, rather than not get her. No, no, he would not sell her then. She was a beauty—a picture—a doll—one of the regular bloods—none of your thick-lipped, bullet-headed, cotton-picking niggers—if she was might he be d--d.