Credit against advance payment

Hi friends,

I have an situation where the customer's are to be delivered / order created only if the advance payment is received from the customer.

There are some set of customers who have a set credit limit upto which they can procure from the company, for which i have set the credit management, and also there are some customers who are not having any crdit limit with them and should be delivered only if they make payment in advance.

Pls advise..

Regards,

Aruna