Hi Josef,
The Functional cosultants in the Team should be able to answer this as to how the integration has been done and the respective tables.
In case there is no STD datasources then you can build a view of Generic Datasource based on the Tables Recommended by the FC Team.
Also check how Fundcentre is mapped in MM module,like it may be same as Profit centre or Plant.
In that case get both the data in BW and build a MP/IS and report for both the KPIs...Amount Alloted and Amount Spent ,if drill down is require then show the POs.
rgds
SVU