I have a librarian who is offering me a mending book for 10 emeralds as well as a feather falling II book for 1 emerald. If I click on the mending book option with less than 10 emeralds, I can still trade but I get the feather falling II book for the one emerald. It even shows up in the trading slot as a feather falling II book even though I chose the mending book option.