To return a loaned or borrowed product back into inventory you will want to create an adjustment for that.
Click on Inventory tab
Click on Adjustment
Click on Add Adjustment
From here you will want to choose the Adjustment type “Other” and put in the description “Returned product from loan” or whatever is going to help you know what it is. Then click Save
From there you will want to list the product and the quantity that is being returned into your inventory in the Change By Column
When finished click on Apply Adjustment in the top right.