Offsetting PO line items

I have not found what I consider an acceptable way to transfer inventory and cost it at the same time.  The solution I would have liked to have would be to have positive and negative PO Lines and then the ability to do a landed cost as well.  Then when I transfer 20 pallets of XYZ I can put PO line to the 3PL vendor for the new cost of the items that we will incur from them when they receive the stock.  (eg. receiving fees, put away and initial storage) placing the inventory now in that designated warehouse for fulfillment of related customer orders.  Then I add a negative 20 pallets of XYZ for the current cost of the items and not their location etc. (we use produmex).  Then if I use a seperate freight company to transfer the inventory it can use the landed costing to top off my unit cost at the 3PL location.  I have fiddled in test data but it will not allow me to have a negative PO qty.  I have used another ERP solution in the past that did allow me to both have postive and negative units on PO and then they netted to the ultimate vendor invoice I was going to pay for 3PL fees.  And then of course pay freight company if different using the Landed costs.  Seems like the cleanest way to do this surely it can be done somehow?