Drawing an entity-relationship diagram for a store business context
First of all: (A Cashier can also be a Customer) 
In this case, you can use a 1-to-1 relationship named "CAN ALSO BE" between Cashier Entity and Customer Entity. And also transmit Customer P.K as F.K to Cashier. This F.K (Foreign key of Customer in Cashier) can be null too and should be UNIQUE. 


Edit: (based on @RDFozz notice) 
We can use the similar User_ID in both Cashier and Customer to show that a Cashier can also be a Customer. 

Secondly: (Only a Cashier can add a Customer to the Store's database) 
Meaning that each Customer has a creator and should be a Cashier. So you have a one-to-many relationship named "CREATOR" between Cashier and Customer. So, you should transmit Cashier P.K as F.K to Customer. This F.K (Foreign key of Cashier in Customer that shows creator id) should not be null. 

Thirdly: (Only a Cashier can add a Customer to the Store's database) 
We can not put ALL Business Logic of our project into ERD. For this reason we creates Business Layer or Business Logic Layer (BLL) or sometimes we write some Stored Procedures or Functions with Enhanced SQLs (those have IF, Loops and etc.) 

If we put P.K of Cashier as F.K into Customer and prevent it to be null, that's enough. Your Insert into Customer should has a Cashier ID to be done.