Returnable Inventory Items

Our manufacturing company sells juice to wineries and uses 60 gallon barrels and 275 gallon ibc's to deliver the juice to our customers.  These containers are listed as inventory parts.  We sell the barrels at $40 a barrel and sell the ibc's at $90 an ibc.  Our customers can receive those amounts back that were charged to them if they are returned cleaned and with the plugs.  My current issue is keeping an accurate inventory count on these containers because I'm unable to create another inventory part listed as container returns because you cannot create an inventory part with a negative price.  Currently these returned containers are listed as different line items as other charges.  So when a container is returned it is not added back to that container item listed as inventory part and there is no accumulation of the returns under the other charges item lists.  How can I make this work where all the containers (Inventory reduced when sold) and returned containers (Inventory added back when returned) are accumulating as one item so I can have an accurate count on inventory for our containers in QB?