How to limit the number of rows in select statement for a condition in sql server 2012
It is difficult to say without your current query and sample data, but looks like this is something you want

select t1.*
from 
(
select t.*,
    row_number() over (partition by Customer,Product_Code order by Unit_Unit_Price) as rnk
from 
 (your current query) t
) t1
where t1.rnk=1


Note: If your current query doesn't have any group by then you can plug in row_number() over (partition by Customer,Product_Code order by Unit_Unit_Price) as rnk in your current query select clause and filter it in outer query as the example above.