model new fact

Anonymous  are you sure you have the relations right? to be more specific, is it really Product-Balance and Product-Ext M-1? I would guess that one Product may have multiple warehouses and multiple product may be in multiple warehouses, thus it should be M-N.
If your drawing is correct, I would suggest you merge in Power Query  Ext, Ref and Balance to Product so you have just one dimension.

Hi Anonymous 

Please use the Pef Product and Ref Balance many to many relationship and Ref Product filter Ref Balance
or
Marge Table as New in Power Query and create a new table for Fact_Stock

If solved your requirement, please mark this answer as SOLUTION.
If this comment helps you, appreciate your KUDOS

Thanks
Pijush