Japanese readers are amazingly lazy when a fantasy world currency needs to exactly match the yen in so many isekai works. I suppose superficial world building is one reason why light novels will stay light novels.
Imma bet the old lady was a world traveler similar to him. She foretold a summoning and waited out there to check up on them.
If you read further, there's these facts to consider:
summoning magic is a secret art bc past heroes from another world are never mentioned again (although demons know of the magic), word magic can change your appearance, it is also like a script language for the world, thus, she can technically ask true or false questions from the world to find out about his summoning (or also change his status to protect him (the kingdom must have sent someone to grasp his nature bc this is basic protocol if it were me) and hide the fact who's magic it belongs to); she can also check people's status and make the magic invisible (ex. Appraisal Invisible.) so that she could identify him[/spoiler ]
MC asks "how did she know I'm from another world?" when he's wearing a completely foreign style of clothing likely consisting of synthetic fibers. What makes this worse is that they're probably going to try to pass off the MC as super smart.
I'm a bit late to say this but, even with his clothing it should be a wild guess that he's from another world cuz his clothes fr could pass off as a noble's clothing or from some other country, no?