Hello David,
While posting the Inventory adjustment journal
For -ve quantity the system will always post with the average cost price at the time of issue.
For +ve quantity the system will post with the value that is keyed in in the cost price field.

You can enter a default cost price in the released product master,

Let us say, We created a new item, Mentioned the default cost price as 10.00 (Latest cost price is un-checked)
create a few PO transactions
PO 1 :- 1 @ 10
PO 2 :- 1 @ 15
Hence the on hand value of this items comes to (10+15)/2 = 12.5. (this is the cost price mentioned in the inventory on-hand screen )
Now , If we try to adjust a quantity,
If we make an entry of +1 or -1 cost price will default to 10.00 ( picks from the Released product master)
If the quantity is +ve, the inventory transaction will also be posted with the value keyed in the adjustment journal ( in our case +1 @ 10 )
If the quantity is -ve the inventory transaction will be posted with the average cost price ( in our case -1 @ 12.5 )
Thanks, Kaushik
This post is my own opinion and does not necessarily reflect the opinion or view of my company or Microsoft.
P.S:- Please mark the answer as verified if has helped you to understand the system behaviour.