"The man reeks of innate magical abilities. I'm not sure the scope of them, but anyone doing you a great favour in exchange for only a strand of hair certainly knows what he's going to do with it. Do not fool yourself, Verity."

The funny part is he's the same way. Maybe that's how he recognizes it. He'll make innocuous requests in exchange for things, and then you find out what you gave him was more valuable and dangerous than you could ever realize. Takes one to know one!