I don't think it is as simple as that. I mean from a practical standpoint, that might make sense, but think of it from an emotional standpoint. She broke the other three knives that he gave her during the demon fight, and she would see it, meaning the cursed/magic weapon, as proof of his faith in her.
Kinda glad this dude's an actual asshole. He's not doing the overplayed "I was being mean to test you" and he's actually discriminating against slaves.
It's the opposite. He assumed, probably from previous experiences, that MC would force his slave to sacrifice her life for better combat power and was appalled at the idea. That's why he was so stubborn about refusing the job.