What is the difference between slavery tolerated because of necessity and slavery affirmed as a moral right?

Tolerated b/c of necessity - you don't really want to, but you think you have to. Let's say you have a farm, you need people to work it.

Trouble is, it's not making enough money to pay the farmhands decent wages. The solution is slavery.

Now you don't need money for wages, as long as the farm is making enough to keep your slaves fed and clothed, you're OK.

Slavery as a right - that's when you think it's your right to be master, and that they deserve to be slaves.

In this case, you don't think there's any unfairness in the situation.

You're doing what you are set to do, and they are doing what they are set to do. Like when you expect a dog to come when you call on it.

Or when you expect a waiter/waitress to be attentive to your needs in a restaurant.