Hi Lucas,
As per trace information, custom RFC FM has designed such way like, if one of quantity missing then it has to return error message as below.
<item>
<TYPE>E</TYPE>
<ID>Z1</ID>
<NUMBER>999</NUMBER>
<MESSAGE>Fail: Missing origin quantity.</MESSAGE>
<LOG_NO/>
<LOG_MSG_NO>000000</LOG_MSG_NO>
<MESSAGE_V1>Fail: Missing origin quantity.</MESSAGE_V1>
So ask SAP Technical Consultant (Abaper) about what validation he has keep in Custom FM and pass accordingly.
And there wont be any problem from SAP end as of my knowledge, because XML will generate based on RFC FM Import, Export and Table Parameters.
Onemore important point regarding RFC FM is
1. FM Import & table Parameters are visible as Request in Jco Action block
2. FM Export & table Parameters are visible as Response in Jco Action block
Regards,
Praveen Reddy