Odata API API_MATERIAL_DOCUMENT_SRV throwing error Deficit of PU GR quantity 20 PC (M7021)

We have a particular case where we post a Goods receipt against a Purchase order (Movement 101) after the goods receipt we post a Return Delivery to vendor (movement type 122)
with reference to the PO without any issue. However if we repeat the steps and try to post a second 122 movement we are receiving error 'Deficit of PU GR quantity 20 PC ....' and the posting fails. In post goods movement MIGO it works fine.