So you want the player to be able to drop items on a page then come back to that page and pick them up?
If so that's not how it works. When you drop an item you need to go back to the page you first got it in order to get it again.
If that's not what you want then I don't really understand the point in only letting them drop it after a certain point. You can either make it droppable by default, so the player can decide what they like and keep them or you can not make them droppable, and either have the player keep all the items or write a script that removes certain items after a certain page.
Only alternative I can think of is the following:
- Have a page with links to all the items they own.
- Have a page that links to all the items the don't own.
When they click a link on a page, it either adds that item to their inventory or removes it depending on the page. In order to access those pages you would either:
A) Periodically bring them to the pages so they can decide what to do (or to do nothing)
B) Have an item that links to the pages so they can access them at any time in the story
Though all in all unless you have like 15+ items it seems like more effort than it's worth.