Eventhough you have the focus that was stolen, he doesn't want it. He always takes YOUR focus instead, the one you had made by the smith in Tenebrae and enchanted on the altar. Kinda funny bug in the game I guess.
So if you don't have your foci with you, have them in your backpack and talk with him again. Don't worry, the stolen focus is the same so you can cast from him afterwards.
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