In this video, a standup comedian is in-character as … his character, working at a food truck. A customer asks him if the Smokey BBQ Pork is halal, and the character says “Yes.”
An adult Muslim should know that Smokey BBQ Pork is not halal, but putting that aside for the moment. In Islam, if you wind up doing something haram due to deception, are you still considered to have sinned?
I was just talking to a very devote Muslim and though a bit off the main point, perhaps something can be gained in this context. The question of if he could eat pork if there was no alternative and stay halal, and he said yes as halal is a act of submission to Allah, and if there is truly no alternative (and one is not required to die to stay halal, actually quite the opposite), you give thanks and eat, and he indicated the giving thanks is what is needed to make it halal in those circumstances - or that is my basic understanding of how he explained it. I would assume something very much like that in your case, if the person actually did not know is was not halal and thought it was, it is still halal.
It’s akin to keeping kosher. The sin lies in intent. kanicbird has it correct as I understand it.
If you are deceived, its not your fault.