If I understand correctly, you customer wants to use a punch-out catalog to purchase goods, but does not want to process the Purchase Request, the Purchase Order, nor the Receipt in Ariba - but in their local system (ERP or else).
If my understanding is correct, then this can be accomplished using Ariba Procurement Content (APC).
APC is a solution that allows users to connect from the customer's local system (ERP) into Ariba (punch-in) to choose items from a catalog, and once the goods are selected, the users are invited to check out and continue the purchase request in the local system.
Hope this helps. Let me know if you have any more questions.
We (http://www.Punchout2Go.com) are working on a similar scenario. Buyer enters site and submits RFQ, Supplier then builds the quote and submits. Buyer receives an email with a quote id. The Buyer then comes back through punch out and submits the Order ID received in the email to pickup the quote and punchout the order. We are doing this with Magento Commerce and can work with you if you have not yet developed the solution in place or if you require other types of services related to punchout. We also have another similar solution we are implementing with our partner, Hubspan, where the cart orders are passed through Hubspan in a similar manner as described. Whatever your exact requirements are, we could likely develop for your needs either directly or with Hubspan.