OPDC API error code

RETENTION_INVALID

HTTP status 422

An AP invoice's retention is negative, more than the invoice total, or set on a credit (a negative total). 422. Nothing is written.

The API answers with a problem document (RFC 9457) whose type is https://twinn.engineering/errors/retention-invalid, and with X-Error-Code: RETENTION_INVALID. Branch on the code, never on the detail text.

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