The people who were exploited should be the ones to benefit from those diamonds.

i mean probably, but this would be a question of what the law says. If we're talking philosophy that's irrelevant here, but to include it anyway, it would be something like "the most ethical source of any given item should be most preferred over any other source of said item" or if we're operating under an ideology of anti-human exploitation idk where we would even start. You need a really concrete definition of exploitation, and how to combat it effectively, without just exploiting more people.