Building a procurement function: Impact on fraud risk - why lower?

There is an organizational perception that centralizing procurement authority and changing procurement delegation from a management entitlement to a personal authority (i.e. that most spend needs to be managed professionally through procurement, and line managers can no longer enter into commercial contracts as they see pleased), the overall organizational fraud risk will increase. I am struggling to counter this perception which then blocks discussions on creating a central procurement authority. Any ideas/research that can dispel this perception? Please feel free to reach out privately. Thank you.