2974296 - Deficit of PU GR quantity in API (M7021)

When creating a Return delivery to vendor (movement type 122) via the Odata API_MATERIAL_DOCUMENT_SRV you are running into error message M7021 or error message M7328 and the posting fails.

SAP S/4HANA Cloud Public Edition

Post a 122 movement via API_MATERIAL_DOCUMENT_SRV

GR based IV case, missing the required reference fields in the payload. 
If you have a regular Purchase Order Item with a stock material, you can post a 122 return via the API with reference to the Purchase Order. 
If you have a regular Purchase Order Item with a stock material and with the 'GR Based IV flag' set, you can post a 122 return via the API but you must pass the assignment block reference as well as the Purchase Order.

In the payload, pass the corresponding assignment block fields: 
...
"InvtryMgmtReferenceDocument" : "xxxxxxxxxxx",   (mseg-lfbnr) 
"ReferenceDocumentFiscalYear" : "xxxx",                  (mseg-lfbja) 
"InvtryMgmtRefDocumentItem" : "xxxx",                  (mseg-lfpos) 
...

See KBA 1969463  
See KBA 2890266

M7021, M7328, VFBL, EKPO-WEMNG, MB_FILL_EKPO_ENQUEUE_TABLE, FM07MEM0_MENGE_PRUEFEN, MB_CHECK_MATERIAL_STOCKS, WEBRE, GR to vendor, 122, Deficit of PU GR quantity, API, A_MaterialDocumentHeader, 101, GoodsMovementCode, Reference to Material Document , KBA , MM-IM-GR-PO-2CL , Goods Receipt from External Procurement (Public Cloud) , MM-IM-GR-PO , Goods Receipt from External Procurement , Problem