select 
  cast(4 as numeric(15,2)) - cast(5 as numeric(15,2)), 
  cast(5 as numeric(15,2)) - cast(4 as numeric(15,2))
from rdb$database

             SUBTRACT              SUBTRACT1
===================== =====================
                 0.00                  1.00

             SUBTRACT              SUBTRACT
===================== =====================
                -1.00                  1.00

 OUTPUT 'test.txt';
 select 
  cast(4 as numeric(15,2)) - cast(5 as numeric(15,2)), 
  cast(5 as numeric(15,2)) - cast(4 as numeric(15,2))
from rdb$database;
 COMMIT;

                  -1                    1

select
  c.Customer_Id, c.Account_No, c.Debt_Sum old_sum, a.Debt_Sum new_sum, c.Debt_Sum - a.Debt_Sum dif_sum
from Customer c inner join customer_a a on (c.Customer_Id = a.Customer_Id)
where c.Debt_Sum <> a.Debt_Sum

select
  c.Customer_Id, c.Debt_Sum old_sum, a.Debt_Sum new_sum, c.Debt_Sum - a.Debt_Sum dif_sum
from Customer_B c inner join customer_a a on (c.Customer_Id = a.Customer_Id)
where c.Debt_Sum <> a.Debt_Sum