| I had the same problem. The trick is not to free any slaves until you deliver the ledger to Garisa. When you get the quest, he specificaly says not to undertake any action, just get the evidence. After you give him the ledger, he will ask you to stop the mining operation. Then you can free the slaves.
I was in the same position you are in. I think you have to get back to a saved game in which you haven't freed the Caldera mine slaves yet.
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