I used to play The Dark Eye (Das Schwarze Auge) myself and in my opinion it strongly resembles medieval Europe - apart from the obvious fantasy additions. It's rule books give information of the economy, trade and demography of nearly everything.
Therefore the dilemma is quite understandable; in medieval Europe a sword was worth a fortune and in the world of The Dark Eye it is the same. However, a hero needs a sword right?
Well, maybe not necessarily. In our group we don't have a rule to handle wealth but here are some thoughts on how we somehow deal with it usually (after writing it down I realised that our heroes are not that wealthy because of these guidelines so it may not suit your question that well):
Equipment has flaws. Nearly everything a (starting) character possesses is not a shiny new blade but rather a crude axe, and old dagger from the grandpa, a warm winter coat with some holes, the elven bow that was handed to the character in a ceremony and would not be considered for sale.
Producing goods has its cost. It is true that potions or magical items are worth a hell lot of money in The Dark Eye. But producing these is an investment and requires material and knowledge - both of which can be costly to find.
Valuable findings are rare. Again, nearly everything a character finds, loots or gets possession of has either flaws or is an unknown object that requires identification.
However, here you'll find yourself in a situation where player and character knowledge interferes. An adventurer may not have ever seen a telescope and finds no use in keeping it but the player knows it is worth a ton of money. We also have some problems with stealing because the players have knowledge about the worth of goods - also stealing creates a difficult situation by either letting it pass or punishing the character that may hinder adventure progress (but that's a different story).
Carrying gold or whatever currency is not very common. Carry a chest full of gold and silver and it will attract thieves. Taxes may be imposed upon crossing borders and cities. And the greedy tavern keeper will charge more if he spots the characters entering in bling bling.
Value comfortable but rule-speaking unnecessary goods. Create an environment where player not only value the next best weapon or armor because it will raise their damage and defence in battle. Good shoes can be expensive but keep from getting exhausted and have a minor impact on game balance. The same with regular feel-good massages, company during the night, the feast at the end of an adventure. The more characters act with each other and the more a personality they have the more readily the spend money on that. All these are sorts of money sinks.
But in the end the characters are heroes and it may be frustrating to not get a bit wealthy at some point. The reward for risking their life is loot, treasure and occasionally - a very special event - the acquisition of a new and shiny sword.
Questions
Well, the easiest way is to have your character ask or empathize with other characters in play. "This war has to be pretty hard on you. Weren't you a civilian before?" These work well because they can be a chance to roleplay your character and ask valid questions of theirs.
Some players get stage fright though, so be mindful of that and willing to speak out of character ("Hey, it's cool. I was just hoping to give you a chance to spotlight your character a bit.")
"Remember the time?" loaded questions
This is a fun way, but requires the players to have some trust with each other. You ask loaded questions to them that serves as a starting point, and they take it as true and build off of it.
"Wait, you're not THE 'Nebula-Smasher' Johnson? How DID you escape the Queen's fleet?"
"Is this going to be like the last time you got us BOTH arrested? You still owe me..."
Players who are not good at improv will not roll well with this, and you may want to talk out of character to let people know you might do this.
Scenes and focus of play
This requires the whole group to buy in on it, but it works in a lot of games. "Hey, I'd like to have a scene where I can talk to (fellow PC) about the fallout from these failed negotiations. Can we do that?"
Openly requesting and declaring scenes makes it really easy and fun to set up scenes about beliefs, personality, history, and thoughts on the situation. It lets everyone know what you're interested in, and to spend time exploring characters specifically.
Best Answer
I am going to be honest here, first of all. This is a problem for me too, because I play with a bunch of people who are really into STEM stuff. I’ve been on both sides of the player/GM line around this issue, so as a player, I can say this: it is really hard to turn off a part of your brain that thinks about problem-solving, especially if that’s part of your job. Honestly, I would try to avoid blaming your players for thinking about what they would do if they, not their characters, were in this situation.
The way I see it, you have a couple of things you have to do here.
What to do
First, talk to the players in question. I think there might be a difference in opinion. To me, it seems like your players want to do semi-realistic stuff using OOC knowledge. You seem to want that to be justified in character (please correct me if I’m wrong).
When I was in this situation, I talked to the player in question and tried to figure out a couple of things. One, is creative problem solving a part of the game they like? Two, is it a part of the game you like? Three, you say they don’t have skills appropriate to the task. How would they (this needs to come from them, preferably after a discussion about the first two things) work it out in character? Where have they heard about the idea, tried something like it, read a schematic? This can be great for worldbuilding. My suggestion for this is that you be open to ideas from the player, but veto something if it’s all the way out of left field, like a hermit-like character who’s never been in a city coming up with a plan to build a ten-story apartment building.
A note: This discussion may take a lot of time or a little. In my first example, it took about five minutes at the end of lunch break, while the second example took about ten minutes on a video call after the session, but I have to deal with something similar in a new campaign as a GM (but can’t talk about it yet because it isn’t resolved) and it so far has been twelve email messages with no resolution. If there is a way to have the talk in person or when you can see and hear the player (video call), that would be best (it may not be possible, given COVID-19 concerns, but you need to have body language and tone of voice clues as much as possible when trying to compromise).
I would, after you talk to the player individually, also talk to the group. Ask the same questions and then you and the player together present any solution you came up with for group approval. It may be useful to talk to some players individually before or after if you don’t think they will agree.
The second thing to do is pretty much the same as the other answers. If there isn’t a logical reason for a character to make that plan work, make them roll for it or veto it. Maybe the INT-dumping barbarian actually has a really great idea once in a while (no insult meant here, I thought of the logical extreme as an example).
Examples
This is an example from one of my games where this worked. I sort-of-kind-of co-GMed back in middle school with a friend for a world we built together. One of our players, playing a noble (high INT, low WIS, and no survival skills, in a homebrew ruleset), said she wanted to make a shelter for the wilderness adventure they were on according to plans she’d drawn up before the game.
The player, call her A, had a good bit of wilderness and engineering experience but the character really didn’t. I talked to A about it between sessions (at our next break between classes), and she said she liked creative solutions, we talked to my co-GM and he was not really sure what he thought about it, then A decided that the character had read some schematics but didn’t know how to do it or whether it would work. At the next session, A rolled to see whether she could remember how to build it, then a different check to see whether it worked right (which failed, but she didn’t know that until it crashed in the middle of a long rest). Another player, who IC had helped with planning did the actual building, and they used it for a while.
As a more experienced GM, I think I would have gotten group input on the decision, since we had a couple others who played the same way. We were young though, and A and I are still friends, so I think it worked out fine.
On the other hand, this is a current example from the player side. I am in a number of campaigns right now, and as a player for a D&D 5e campaign, I am on the other side of the discussion.
For context, I’m playing a formerly secluded cleric (high WIS but INT as a dump stat) in a homebrew setting, who joined the adventurers as a favor to their patron. I’ve done a few things that are similar to your player, like last session I tried blinding a dragon (This was before the flanking thing, by about a round) by using create/destroy water to make rain in its eyes. The DM was not too happy, given that the whole group, but myself especially, is good at coming up with real-world solutions (that work but often are entirely metagaming in a way), to have to try to figure out the effects of my solution again. What he did was to come up with an immediate resolution (it blinds the dragon for a round) and then to message me to say we needed to figure out how to deal with my solutions. After the session, we decided that creative solutions are going to require both an IC justification for the knowledge to see if I have the knowledge (backstory-based, class-based, race-based, or a reason to know), as well as a roll of either a skill or an attribute to put that knowledge into place (plus anything to actually do it). I think it worked out well; he had a procedure to deal with it, and I had the ability to come up with fun solutions.
The one thing we didn’t really do is to figure out what happens if I don’t have a reason to know, which so far seems to be “find a way another party member would know” (although it didn’t come up because so far we’re in the woods a lot and I’m playing a cleric of Mielekki).
Living with the procedure
In my first example, the talking it out individually plan worked, but if it doesn’t work for you, this might be time for a mid-campaign session 0 to discuss it. Get all the players involved when you come up with a resolution or if the two of you can’t come up with anything, and try to figure out a group opinion on this kind of thing and a way to resolve them. Others may like the creative solutions, or hate them.
In the second example, I think it would have been really easy for a different GM to hurt my feelings, so I would be careful with what you say. Don’t be accusative and do try to make sure they get things they want. However, be honest when something is wrong, and make sure you agree with the solution as well. It will not be fun if you can’t live with the rule, I can say from experience.
In general, for your next campaign you might want to include in the session 0 a discussion about OOC knowledge and justifying it in game to stop this from happening again without a precedent.