In large organizations, all service providers and vendors are sent to Procurement to negotiate contracts.
Often, due to information technology, Procurement is empowered to prevent the employment of a vendor and service provider by not issuing a purchase order, which is a precondition to get paid.
Despite the “mission statements” and b.s., Procurement is there to force the price down. The negotiation with them has a veneer of professionalism, but it is all about money.
My experience is that if the manager who has invited you clearly wants you, he will “take care” of Procurement for you, and Procurement will be merely a vendor-registration process. If the client is ambivalent, or if he is buying an OD product and not a service, the OD vendor will be sent to procurement.
In every instance where I was not personally sent to Procurement, the client was far more serious about getting results.
My advice to newcomers is to add 10-15% to your bid to make the folks in Procurement feel good. They will be able to show off what great business partners they are.
Personally, I do not negotiate with Procurement. Routing me via Procurement means that the client is not really interested.