Issue in centralised credit management

Hi

I am using centralized credit management. Here I have 5 customers which are linked to different company code but belongs to same group of companies like reliance group of companies. Here I want to set  total credit limit 10 lacs to all 5 customer, but if I have given 1 lac credit limit to 1st customer, 2 lacs to 2nd customer if i want  to create SO with 3rd customer i must be able to give the  rest credit limit i.e 7 lacs or whatever is available from total limit. how can I configure the same .how should i control FD32 of these customers.

Regards

Prashnt