Well considering the elves killed his brother Marcus Lanik wouldn't have the best disposition towards her in the first place. He's in debt, but he still can hold on to grudges. And also, he's smart. How would it look if a man of his stature bought a slave and immediately let her go? Imperial eyes are on him, and he has a daughter to worry about. Any bad limelight could easily have him accused. I know up until now he's come off with a pretty liberal ideology compared to most Imperials, but I guess I haven't emphasized the fact how dangerous it is for him to be spilling such ideas in public.