Hi everyone,
I think Haresh suggestion is the best because with BPM you won't have any important improvement, you will have the second problem like the ABAPers, and you will include one more point of communication complexity only to do a collection of the data. I think the Proxy-JDBC would be the best option, the ABAP can control with an easy Z table if the have come correctly, even they can do an ALV for the client, if the client want to have a deep control.
If you have a field to difference one invoice set of registers of another, it will not difficult for one ABAPer to develop the calls and to do an automatic retry if it detects any lack.
From my point of view as normal rule (not always) the integration must be the less complex part, letting a better scalability and change facility.
Regards.