Sounds like a can of worms to me. Most people that shop on line do so for convenience. Adding an extra step to the process sort-of eliminates that. Won't be long before your client asks if you can just get the credit card for them too.
Have a look at this:
Processing off line
I imagine you could just remove the payment processing after you build the cart. Might be easier to just recommend they contact their current credit card processor and find out what they can offer in the way of online processing.