Yeah, I think it works in most situations, but not all right now. I have a save right before the hag offers the deal and don't get anything.
If you kill Mayrina before fighting the hag, the hair doesn't show up. You get a dialogue option that says something like, "I'll take the power, the more the better."
But if you go through the normal dialogue options with Mayrina still alive then the hair works. Took me a couple of playthroughs to figure it out.
