This is kind of a weird digression, but...
Dwarf Fortress is one of those rare games where I'd agree that putting realism first is actually better for it.
That said, realistically speaking, smithies are extremely unlikely to just be fly-by-night operations. Forges aren't something you can just shove in your back pocket and ride into the night with, they're built into the foundation of a building you have to have built/inherited/bought/payed advance rent upon.
Hence, outside the (likely extremely rare) instance of a smith that would need to skip town for totally unrelated reasons, the only way someone might pull the scheme of running away with the steel bars would be if the person who you agreed to pay up front for a job turned out not to be a smith, and you never bothered to check that they actually owned a smithy. The "skip town for unrelated reasons", aside from "goblins sacked the city" should again, be pretty rare in realistic settings, because people were not nearly as mobile in the middle ages as they are now. You needed to spend your whole life building up a trade and a reputation in that trade that you're absolutely flushing down the toilet by pulling a grift like this. A reputation in a town was basically the life's work of a craftsman, so most would be horrified to even consider lighting their reputation on fire like this.
In the case that the player does come across a grifter posing falsely as a smith who doesn't actually own their own smithy, there would probably be some tells that anyone who was paying the slightest bit of attention would be able to notice, the way that a vampire stood out for all those human skull ornaments and how if you asked what they do for a living, they say they worked as a ranger for 5 years and killed 600 humans doing it. Basically, this would be a Nigerian Prince level scam that falls apart at the slightest inquiry. In the event the player is duped by pure lack of paying attention anyway, though, it would be fun to have a quest where a player has to track the grifter down and bring them to justice. You could probably have them as a topic of conversation you could inquire about, and a grifter that keeps on the run would probably leave a trail of people laughing and talking about how ol' Urist got one pulled over on 'er by the guy, and he might have gone further west, off to Oaken Pulley territory.
Inversely, the player is an untrustworthy drifter unless they're something like a hearthperson in the town where you're getting the forging job done. (Your lord will probably be pushed to buy the goods in your place, which will make them angry at you, if you didn't come and pay for the work down.) Having a good reputation might mitigate this somewhat, allowing more to be paid later, but some reasonable split on the cost would be made, like half up front, half on delivery. If the PC doesn't come to pick up their sword in a reasonable time (and I'd say a month or even more is probably a reasonable time in medieval land, and that might also depend upon player reputation), the NPC can sell it to someone else to recoup the cost.
Again, reputation was everything to merchants in the middle ages, as someone who was a fly-by-night grifter once would often have to stay a grifter on the run from the law for the rest of their life.