What value are you comparing? Where is this information held?
I compare "Actual" + "Commitments" in SAP to "Plan" in SAP - for each project (WBS element)
In Prosper I simply look at "Aivalaible" at Quick project view)
Can you provide more details of where the error message "Budget was exceeded" is displayed. How is Ariba performing the budget check when approving the PO?