Hi,
I thyink the replication to backend happens by RFC FM. what you can do is take the GUID of PO and run BBP_PD_PO_TRANSFER_EXEC FM. before doing this put a BP in FM B46B_DPO_TRANSFER at code where FM BBP_PO_INBOUND is called. Here you will come to know what data exactly is getting passed to ECC.
thanks,
sankar.