what happens if i buy a slave and he sees me turn into a werewolf

I have never done that but I would think it would be the same as any npc seeing you change into a werewolf. They would tell everyone and everyone would hate you afterwards. I do not think that they being your slave would make any difference.

Save before you do and try it. You can always reload your save.

I think that it would do the same thing as when a normal npc sees you, but i would save first like Sonya Snow-Song suggested.