i need to get all bills numbers and the sum for every bill aloan.
via select customer name and date between 2 dates.

SELECT bill_no, customer_name, SUM( detail_line.amount ) AS bill_total ;
  FROM bill_details JOIN bill_header ON bill_details.bill_no = bill_header.bill_no ;
                    JOIN Customer ON bill_header.customer_no = Customer.Customer_no ; 
WHERE between( bill_header.bill_date, dStart, dEnd ) AND Customer.Customer.no = SomeValue ;
GROUP BY bill_no, customer_name into cursor csrBill Totals

will give you all the bills for a specific customer in a particular date range. If you want all customers for that date range, modify the query like this:

SELECT bill_no, customer_name, SUM( detail_line.amount ) AS bill_total ;
  FROM bill_details JOIN bill_header ON bill_details.bill_no = bill_header.bill_no ;
                    JOIN Customer ON bill_header.customer_no = Customer.Customer_no ; 
WHERE between( bill_header.bill_date, dStart, dEnd ) ;
GROUP BY bill_no, customer_name into cursor csrBill Totals