A New Fence for Fisk in Gothic 1 Remake is completed by first finishing Out of Sight for Thoros, then recruiting Sharky in the New Camp, and finally returning to Fisk for the reward.
How to complete A New Fence for Fisk
Step 1: Clear the unlock requirement
Finish Out of Sight for Thoros first, because Fisk’s quest will not properly open until Mordrag is out of the picture. If Mordrag is still active, go back and resolve that storyline before talking to Fisk again.

Step 2: Trigger Fisk’s request
Go to Fisk in the Old Camp marketplace and walk close enough to start the conversation. He will complain that his old fence is gone and ask you to find a replacement.

Step 3: Travel to the New Camp
Head to the New Camp by the route you already know, or use a helper route if you need one. If you are undergeared, avoid unnecessary fights on the way, because the route can be punishing early on.

Step 4: Check Cipher first
Inside the New Camp, enter the tavern area and talk to Cipher. He will reject the job, but he will point you toward Sharky, which is the real solution.
Step 5: Recruit Sharky
Go into the main cavern settlement.

And find Sharky near the campfire-side housing area.

Ask him to become Fisk’s fence.

And he will agree after the dialogue.

Step 6: Report back to Fisk
Return to Fisk in the Old Camp and tell him the New Camp contact is ready.

That completes the quest and awards 300 XP.

Critical mistakes to avoid
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Do not expect this quest to appear before you finish Out of Sight.
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Do not waste time forcing Cipher to accept; he only acts as the lead-in to Sharky.
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Do not rush Mordrag’s related objective if you are badly equipped, because early combat can be brutal.
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Do not confuse Fisk’s quest with his location-based interactions in the Old Camp marketplace.
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If the dialogue does not trigger, leave the area and return after confirming Mordrag’s arc is resolved.