A purchase-order update that would order less on a line than the line has already received, or remove a line that has received anything. 422.
The detail names each line, the quantity asked for and the quantity received. Nothing is written.
OPDC API error code
QUANTITY_BELOW_RECEIVED
HTTP status 422
The API answers with a problem document (RFC 9457) whose type is https://twinn.engineering/errors/quantity-below-received, and with X-Error-Code: QUANTITY_BELOW_RECEIVED. Branch on the code, never on the detail text.