To get him to vouch for your admission to the Shadows you must do a job for him: recover Kalom's recipe (from the Swamp Camp), with him fronting nuggets only after haggling. He trades and trains.
Old Camp, Outer Ring — Chapter 1: An influential Shadow who is also a fence/trainer.
Dexter can permanently train you in Dexterity for Learning Points in Gothic 1 Remake.
Quests Dexter takes part in across Gothic 1 Remake.
Items Dexter sells in Gothic 1 Remake.
Dexter can be found in the Old Camp (Sect Follower).
Dexter: You look tense ... how about some swampweed, straight from the cultists?
You: I'm looking to join the Old Camp and become a Shadow.
Dexter: In other words, you're trying to prove yourself.
Dexter: Hmm ... yeah, there's something I could use a hand with. If you do well, I'll put in a good word for you.
You: What do you need?
Dexter: There's a guy in the Swamp Camp who goes by the name of Kalom. He's a "Cor," meaning one of the top dogs.
Dexter: He's also an alchemist, and he's got a recipe for a, how should I say, "potent brew."
You: What's stopping you?
Dexter: Kalom. That old bastard won't talk to anyone who isn't a cultist. That's where you come in - head to the Swamp Camp and act as though you wanna join them.
Dexter: Kalom checks all the novices sooner or later. Once he talks to you, get me the recipe.
You: I might have to buy that recipe. I want fifty nuggets in advance.
Dexter: Forget it.
You: Fine. Forget about it.
Dexter: Wait!
You: Sounds good.
Dexter: Okay, guess I can spare that much. Here. But don't you think about crossing me, freshie.
You: Sounds like a tricky job. What's in it for me?
Dexter: At some point, Diego will ask me what I think of you. Take a guess what my answer's gonna depend on.
You: All right. I'll see what I can do.
You: No. Give me the nuggets, or get it yourself.
Dexter: Alright, then. If you don't wanna do this job, I'll find someone else who will.
You: No nuggets, no recipe.
Dexter: Then you pay for it now, and I'll pay you back once you have it.
You: I have your recipe.
Dexter: Show me.
Dexter: Seraphis, swampweed, crawler ichor ... hmm. That's the big secret?
Dexter: Whatever. The Fire Mages will know what to do with it - probably get hammered on it, 'cause that's all they're doing these days.
Dexter: I'll tell Diego about this. And here - for your efforts.
You: What's "swampweed?"
Dexter: Green to the core, huh? You can smoke it. Takes the edge off.
You: Show me your goods.
You: How do I get to the Swamp Camp?
Dexter: Just head out through the south gate and keep going east. Maybe get yourself a map as well.
Dexter: Come to think of it, I have one in my stock ...
You: The south gate ... that's the one by the fallen tower, isn't it?
Dexter: That's right.