> It is not possible for all people to have all freedoms. My freedom to go where I want is limited by your freedom to decide who comes on your property.See, this is the sort of contradiction inherent in the "positive rights" worldview. Positive "rights" are always in conflict, which is very convenient when you're looking for an excuse to pick and choose which rights other people have and not very useful as a framework for a stable society.
Negative rights, on the other hand, never conflict; there is really only one fundamental right, which is self-ownership: the exclusive right to decide how yourself, and by extension your property, will be used. The only actions are out of bounds are those which would infringe on others' rights of self-ownership. From this you can infer other rights like the freedom of speech, freedom of association, the right to privacy, and the right not to be enslaved against your will, and together with others you can cooperate to provide each other with things which, while desirable, are not rights, such as food, shelter, defense, gainful employment, and healthcare.
> Meanwhile, large numbers of people are currently being forced into slavery - 20.9 million, by one estimate.
And I agree that this is wrong. The key difference is that these people were forced into slavery—it wasn't their choice. Obviously it's not a very attractive option under any circumstance, but one can easily imagine situations where the alternative might be worse. If you need what someone else can provide, and have nothing else of sufficient value to barter for it, giving up your freedom might be a price you'd be willing to pay. No one else should presume to take that option from you.
> Restricting the rights of some small number of hypothetical would-be slaves in order to protect the rights of large numbers of real people in forced bondage is clearly a worthwhile tradeoff.
Putting aside the fact that it isn't your right which you're trading away, and consequently that this isn't your decision to make, it doesn't actually protect anyone. A person who was coerced into such a situation could simply say so, forcing the other party to prove that they had agreed to it in exchange for some form of consideration.