You can start with a date table which will have one row per day in your table. Then you can add 2 column sto your main table which just has the day (not time) of when an item was check out and when it was checked back in (this is not necessary if your table only records the day it was checked in/out, but not the time). Next you would take the date table and join it with your invintory table on the day it was checked in, and in a seprate copy of the date table join on when it was checked out. Do a table expansion on each table, and add a new column inventory_quantity_update to 1 in the check in table, and -1 in the check out table. Do a union on both table, followed by a group by summing the inventory_quantity_update to get a table with the per day change of quantity per item.
Next, you need to calculate a cummalitive sum on the inventory_quantity_update column (make sure your table is sorted by date). While you can do this in Power query, it is much easier to do in DAX at the visualization layer using the quick measure Running total.
