Hi there!
I'm glad you like it! ^^
To lose an item, you just need to set a negative number on the amount argument. Like -1. This will make the party lose 1 item.
Thank you for your answer, but I'm also an experienced RPG Maker. Of course, I thought I would lose one if I typed '-1', but when I actually tried it, I didn't. It's supposed to be normal. I've already completed the event in a passive way, but I'll try again if I have another chance to use it. Thank you.