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You can pay an extra 200 for his trip to Baldur's Gate.
After releasing Oskar talk to him to learn more about his story. Pass a persuasion check of 15 can lower the price (600). Talk to Oskar and his owner Brem, he will ask 1000 gold for releasing Oskar. If you failed, you can still free Oskar by pay to Brem. Talk to Brem before talking to Oskar, and pick "You have a 'pet artist?", then "'Protection.' It sounds a bit more like kidnapping." You can then try to persuade Brem to free the artist. You can reach the Zhentarim Hideout by complete the quest Find the Missing Shipment.
He begged us to help him escape, promising that his patron will reward us handsomely.
We met Oskar, an artist captured by the Zhentarim. Brem agreed to sell us Oskar for a hefty sum of gold. It took some convincing, but we got Brem to lower the price. Caught up in a romantic scandal with a patriar of Baldur's Gate, he'd fled his engagement to her. Free the artist can be acquired during Act 1. Free the artist is a Quest in Baldur's Gate 3.