Solution : https://service.sap.com/sap/support/notes/378883 (SAP Service marketplace login required)
Key words : 
additional key words sis, mixed business transactions, mixed business transaction, returns items, standard order, open order, solution implement, attached correction, correction, prerequisites
Related Notes : 
       
| 204130 | SIS update: Collective note | 
| 140982 | Open orders for returns; mixed business transactions |