journals not applying credits to inventory account

I have been trying to correct the way that the cost of shipping was applied to a purchase order.  It was applied by unit, but I wanted it applied by weight.  I thought to do journals right in the PO, but the system would not let me put my inventory account into the credit field.

I next tried an inventory transfer A to B and then B to A to apply my journals.  This worked - in that it allowed me to enter the inventory account into my credit field.  However, when I look at the values of the items in the financial tab, or when I look at the cost per item in my product availability window, it is clear that only the debit applied but not the credit.

So this begs the question, how do I reduce the cost of my inventory with an inventory account credit?

I have added some screenshots.

1- not able to input inventory account into credit screen, this was during the purchase process (I thought to credit $x per unit, and then debit $x by weight and that would have finished the problem in one step).

2- transfer to out of inventory to credit the inventory value and reduce its cost

3-  transfer inventory back into correct location to debit it for correct shipping cost and add small amount of cost.  You can see that, even though item 2 above occurred first, the inventory is valued the same at this point as it was at step 2.  The credit did not change its valuation.