We are not sure what is exact issue which you are facing in SAP System. But as we understood from XI, in XI logic currency code is mapping with "ProductSublineitem" field. If we change the currency field map from "ProductSublineitem" to "ProductLineitem" the issue could be resolve in SAP system as from SSN we are receiving currency code in "ProductLineitem".
Regarding SAP side changes, at this time we may not commit to do not required any changes. If you feel, the issue can be resolve by map the currency field with "ProductLineitem" in XI logic, we will do and let us test in Quality environment and we can take decision whether changes are required in SAP or not.