Inventory History has stock levels however doesn't have any Purchase order information. Those values happen to be in a different cube - Purchase Orders cube. We can query one cube from another using LOOKUPCUBE calculated measure function:

1) Create new report using Inventory History cube

2) Add SKU to Rows

3) Add Last Stock Level to measures

4) Create new calculated measure

LOOKUPCUBE("Purchase Order", 
[Purchase Order Fact].[Status].&[OPEN],
[Inventory Products].[SKU].&[" + [Inventory Products].[SKU].CurrentMember.Properties("Key")+"]

String expression syntax  
LookupCube(Cube_Name, String_Expression )  

The actual expression is a standard MDX expression that is essentially evaluated for every value (cell) of the report, therefore we can inject values (keys, names etc) from our current cube query into it. In the example above we are injecting Key for the SKU.

5) add newly created calculated measures