I am not able to track the customer which was removed by itself.
If you view those sales invoices, what do they show? Is the customer shown?
No only the description of items taken by a customer
Then you should edit those sales invoices to include the customer. I believe you either neglected to enter the customer at the time the sales invoice was created or edited them later to eliminate the customer, because Manager will not let you delete a customer for whom a sales invoice already exists.