I left out the whole OP part about "slave". To me that is totally different. Whereas whore and slut are similar, leaving out the money difference, you might call someone either and mean close to the same thing, as either an insult or a playful expression of love or lust, calling someone slave is a deeper level of commitment. Especially "my slave" : it means ownership, possession, full time commitment, although it does not imply exclusivity or monogamy to me.