Because she doesn't own them anymore, and he will get an extraordinary amount of land, money and power out of it. It's not like he's doing it out of the bottom of his heart or because he actually likes her (at this point, anyway)
@Rino tbh from the way the ml said it, it seemed like his problem was less that she owned slaves and more that she whipped them, so slavery might be acceptable in their society (I'd love to be wrong about this tho)