https://en.touhouwiki.net/wiki/Curiosities_of_Lotus_Asia/Chapter_36
In Curiosities of Lotus Asia, Sumireko works for Rinnosuke while she visits. As payment, he gives her really old 1000-yen bills. He explains that such bills are worthless and only used in Gensokyo as hard-to-reproduce promissory notes, and they don't care about the amount written on the bill. According to him, Gensokyo works on the bartering system, so they are more like easy to carry "credits" than anything (if you are familiar with Canadian history, the HBC used a similar system before a standard currency was established back in the 17th century). It's implied she exploits this by offering to work for him in exchange for the bills, taking them home and selling them as collector's items due to their age.
That said, As stated above, mon must exist as per not only the cards in Th18, but Komachi's spell cards also use them, and as a ferrywoman for the Sanzu, she does take payment in the form of said coins. How exactly they are used in Gensokyo is unknown as far as I know.
