Credit limit for Group of customers in sap business one

Hi All

I have a customer with 7 sister concerns. I want to set credit limit for all these customer as one customer.

I have created all the 7 customers as Seperate BP & created a UDF in which I have linked the main customer.

So if user is creating a AR invoice against any of the customer & if credit limit exceeds for the main customer.. Approval should be provided.

How to set approval for this kind of scenario?