I hope she won't stay a slave for long. He obviously pities her, but I don't think he believes that--or is actually--he's in a position where he can release one of the few assets he has.
I wonder if I'd have done the same in his position. If he's gonna go cold and ruthless, he might as well make a better choice than the diseased raccoon girl. How is he going to care for her in his situation? She's likely nothing but a waste of money, sick as she is.
I wonder if he's only pretending to be cold and ruthless, rather than more holistically taking it upon himself to be so in order to survive. I assumed the latter, but his choice in slaves is odd in this case.
Another interpretation is because the slave trader just told him she's diseased and has a mental disorder and probably won't last long, he was thinking he could make the deal dirt cheap, afford it easily and still learn how to handle slaves before getting one that's actually useful to him.