If your inventory is always in the scene, easy to access, like a bar at the bottom for example then you can make it that when you left click a fuse it will be added to inventory and then you can drag it again from there to use place it in different slot.
Otherwise if you need to bring up the inventory everytime you want to use an item, it wouldn't be the most convenient to the player. In that case I suggest you do it this way:
First make a cursor for each fuse.
Now according to what you said, you want to move the green fuse from the purple slot and switch it with the purple fuse. When you left click the green fuse, add actions:
Change condition 'greenfuse selected' to True (attach greenfuse scene
object to a condition called greenfuse_selected and set to be active when the condition is false.) and now the purple slot is empty
Set the cursor to the 'green fuse cursor'(so it will look like you picked it up)
Then when you left click on the purple fuse , repeat the actions above but to pick up the purple fuse and in the same time you want to add an action to check if you have a fuse selected already:
if condition 'greenfuse_selected' is True
Change condition greenfuse_selected to False (to make the greenfuse visible again)
Move object "greenfuse" to (x,y) - add coordinates to move the green fuse to the empty slot
You can also add conditions to each slot ( Example: slot1_full) and set it to true/false depending on whether there is a fuse in it then add an action to check which one is empty to move the green fuse into it, in that case:
if condition 'greenfuse_selected' is true
Change condition 'greenfuse_selected' to False
If condition 'slot1_full' is False
Move object 'greenfuse' to (x,y) - add coordinates of slot1
When you place a fuse in an empty slot, add an action to set your default cursor back.
Ofcourse I didn't include everything above but it should be good as a start.
That's basically the direction that I would go with this, it will be easier for the player not to have to go to the inventory to switch between fuses.
I did something similar but instead of having the items already in the slots, I made a shelf were I had to pick up the items from, and clicking right click would drop the item to it's original place. But that was more suitable for my puzzle design.
I'm sure it must be easier with Lua scripts, unfortunately I am not the right guy for the job... still a beginner with scripting.
Hope this is helpful to you.