SQL Except Statement and Add in One more column
Here is a sample solution (based on sp_BlitzErik's comment):

--demo setup
drop table if exists dbo.CustomerTransaction1;
drop table if exists dbo.CustomerTransaction2;
drop table if exists dbo.CustomerTransactionDiff;
Create table dbo.CustomerTransaction1
(
    CustomerTransactionId int primary key,
    CustomerName varchar(255),
    PurchasePrice decimal(10,2),
    Quantity int,
    Loaddate datetime
);

insert into dbo.CustomerTransaction1(CustomerTransactionId,CustomerName,PurchasePrice,Quantity,Loaddate) values
(1,'Fred',100,5,'2018-10-25'),
(2,'Bob',200,1,'2018-10-25')

Create table dbo.CustomerTransaction2
(
    CustomerTransactionId int primary key,
    CustomerName varchar(255),
    PurchasePrice decimal(10,2),
    Quantity int,
    Loaddate datetime
);
insert into dbo.CustomerTransaction2(CustomerTransactionId,CustomerName,PurchasePrice,Quantity,Loaddate) values
(2,'Bob',200,1,'2018-10-25')

Create table dbo.CustomerTransactionDiff
(
    CustomerTransactionId int primary key,
    CustomerName varchar(255),
    PurchasePrice decimal(10,2),
    Quantity int,
    Loaddate datetime
);

--The solution
INSERT INTO CustomerTransactionDiff (
    c1.CustomerTransactionId
    ,CustomerName
    ,c1.PurchasePrice
    ,c1.Quantity
    ,c1.Loaddate
    )
SELECT c1.CustomerTransactionId
    ,c1.CustomerName
    ,c1.PurchasePrice
    ,c1.Quantity
    ,c1.Loaddate
FROM dbo.CustomerTransaction1 c1
WHERE NOT EXISTS (
        SELECT *
        FROM dbo.CustomerTransaction2 c2
        WHERE c2.CustomerTransactionId = c1.CustomerTransactionId
            AND c2.CustomerName = c1.CustomerName
            AND c2.PurchasePrice = c1.PurchasePrice
            AND c2.Quantity = c1.Quantity
        );

--verify differences
select * from dbo.CustomerTransactionDiff




| CustomerTransactionId | CustomerName | PurchasePrice | Quantity | Loaddate                |
|-----------------------|--------------|---------------|----------|-------------------------|
| 1                     | Fred         | 100.00        | 5        | 2018-10-25 00:00:00.000 |