Greetings, well i'm curently developing a FF type game, the only dropping action there is is the mob loot, but it might help you, what i have is:
-Create a list and ammount of items variables;
-Check if item already exists in invetory, if it does, Raise the ammount;
i think that with this you can do the other way around, instead of raising the ammount, decrease it. I hpe this helps you.