If one or more Items come back from a reservation early, no problem, you can just check them in.  This will track where all of your Items are, and more importantly allow you to rent out the Item sooner than you would have otherwise been able to.

If you have multiple Items in your rental then the stage will automatically move to 'Partial Check In'.

To check an asset in early simply find the item in the rental ticket and press the 'check in' button for that item:

And here is the type of thing you will see in the planner:

(in this case a 4 bike booking, one of which came back early)

See also 

early returns

worked example of early check-in freeing up inventory

column selector on the Reservation Detail Panel