WIP
Here I list some of the results of our pursuit to make a workaround for returning quest items. The NPCs of Kenshi cannot take items, so some item returning box is needed. Or you can make the item sellable to the quest giver, so they can pay you. Nothing however prevents the player from selling that item to someone else. And one cannot check if an object was sold.
Currently the only somewhat working system (not elegant)[]
NPC can unlock a dialog option based on the fact an item is inside a player inventory. NPCs cannot check if the object is NOT in player inventory, but with careful locking and unlocking dialogues it is possible to default to 'player had this item and now has not' deduction. However the player can do whatever they want with the item, a) drop it on the ground or b) put it in a box. The result is the same, the player can always pick it up again later. So it is important to make the quest item nonfunctional and have no monetary value.
Cannot make a house sellable & with a function[]
The idea behind this was to have a machine that takes the quest item as a material and produces the reward. Cause the player cannot operate a NPC machine, I tested if one could sell the ownership of the machine to the player. It does not work. Putting a function to a house makes it unsellable. Apparently you cannot have two click-events in one building.