When you have the Hero of the Village effect and attempt to trade with a villager and you only have enough for the reduced price, but not the full price it will not let you trade.  You can shift click the item, but as soon as you exit or try to trade again it will convert the trade back to what you paid.  For example.  I had 4 pumpkins and the farmer normally required 6.  I had the hero of the village effect so he was asking for 4.  When i tried to trade for an emerald it would not let me or would convert the emerald back to 4 pumpkins.  When this happened again, when buying a book from a librarian it would only let me trade if i had enough emeralds for the original price.