One issue that arises with cost allocation in global organizations is that T&E does not allow to bill across different controlling areas. The cost center of the sender and the receiver have to reside within the same controlling area if you want to use the standard T&E cost allocation functionality. In addition, you might have a separate SAP HR system from your FI/CO SAP system, which presents another challenge. In any case, you will have to develop a custom solution to overcome this issue, which will involve some ALE if you have multiple SAP systems. If your inter-company financial system is not on SAP or only partially, then you will need to develop an interface to your billing system - if you are carrying out internal cost allocation.