Why exactly is the currency in yen when this is an isekai?
I found it funny. In the vast majority of fantasy manga, we have got a yen equivalent currency bearing some made up name, which makes little sense in the underdeveloped world. However, the authors just do it because they can't be arsed to do the math every time in their head when they write the characters shopping and on the other hand, the authors also believe most of the (Japanese) readers are so stupid they can't understand anything but the value of yen.
This manga is funny because apparently the author realised he's not fooling anyone by inventing a new fantasy currency that mysteriously just happens to have yen's value, even though it makes little sense in the context. So, he might as well directly call it yen and be done with it. At least nobody can call him out on having a yen equivalent currency in a sword & sorcery story when it
is the bloody yen.