Bypass fetch data

So here is the scenario:

  1. a customer is created. There is no sales team person saved with customer.
  2. sales order is created. I have set up default to be the user name for sales person. hence, when “New” sales order is initiated the full name of user appears in sales person field.
  3. when i select customer in sales order form, all the fields are refreshed deleting the row (and name) of the user from sales person field.

Requirement:
is there some script or some way that:
a) since data is already in ‘sales person’ field of sales order - the refresh at time of selecting customer does not over-write it.
OR
b) the auto refresh at time of selecting customer does not fetch from the customer to update the sales person field.

any help would be GREATLY appreciated!!