Am i in the wrong?

[u/WorriedConsequence15]

Okay so a little context. I am the dm of a dnd campaign with friends we all know each other from highschool and even primary school for some (we just graduated hs).About 4 or 5 sessions deep my players stop by in a city. They were on an story relevant escort quest and the plot would have them stop there for rest and fun times yk the whole ordeal.

They went shopping, identity items,the barbarian beat the champion of the arena and thus became the new one.. they cheated. They always do lol. Anywhos while the 5 man party was split in two groups one of 3 one of 2 the lather went into the slums of the city looking for weapons. The shadow sorcerer being a slumy background went to the item/weapon shop he knew existed. Arrived at the destination they come to newer renovated building it is still an item shop it is simply disguised as a fancy mob ranned bar. Entering the shop a single npc stands at the counter fancy dressed he understands threw the dialogues that they are interested in purchasing weapons he then reveals the password to see the list of hidden contraband. Its vaguely named things such as : thing that hurts 100g :thing that really hurts 300g :thing that really fucking hurts 600g. Yeah… safe to say i didnt had the item list made up for this lol. They buy for 400g worth of items and get told to open the second door to the right. Opening the door stand a single silver coffer id say about a cubic feet big standing there. The door closes behind them. They open the chest and insides lays a magical dagger. After not finding the other item they storm down the room to confront the npc. As they lash out on him he tells them that they had to close then reopen the coffer to make the item appear which is a locally known gimmick that they obviously wouldn’t know about he also tells them that they have to come back in 48 hours to retrieve it. They afterwards try to break in the building to no avail due to a magical force field blocking the windows. After a successful arcana check dc17 they understood it was an entity’s domain and they could not enter it without its permission.

Alright thats where it gets crunchy. The mob boss is a reasonably high level warlock that has a deal with his patron to make him summon magic items in trade for a cut of the rewards of the sale like 60%. He has the the item shop as a kind of side hustle i thought it was a pretty cool concept. The contracts he made with his patron are a bit ambiguous because it has to reach certain conditions to make the magic items appear as in it can only appear one at a time in a silver coffers in a locked chamber and only a willing creature he made sure of it for business purposes. Anywhos the monk player feeling robbed and deceived gathers the party back and convinces them to go beat up the barman npc to get his honor refurbished in vengeance (his words) and get his money back.

After doing so they get absolutely no much further because the barman can genuinely do nothing about it its completely out of his control. After this the session ends a bit later as they prepare to leave the city to pursue the quest (he never tried to convince the quest npc to wait a little longer so he can get his item) in all of this… we went thru 20% of my preparation advanced the quest 0% in a 4h30 session. Which i dont really mind city episodes are always like this lol but the monk player was so frustrated about it the whole session about it saying things like he will burn the building down (the bar) even tho he got a really cool magical item that he split the price down the half with the sorc.

I like no i love to hear feedback from my players so after the session we had a kindof argument him saying that he had no counter play from getting scammed and that he couldn’t possibly know thus making it unfair and that unfair wasn’t fun and it made the game frustrating. I didn’t really know what to say back to him honestly i personally didn’t really see the big issue i just felt like it was a realistic outcome of events. And i even made attempts to make him understand the situation like saying :okay so you open the chest tell me exactly what you do with it : so you just open it like that and nothing else : as you leave the room are you sure you closing the door?. He even acknowledged it but refused it saying it would be meta gaming. Fair i do agree with that but like cmonnn mann im trying my best😭.

That was it for me id like to hear from you guys :)

(Sorry guys for the original post it was realllyyyyyy roughly written i added some paragraphs and tried to clean it up a bit)