921 Official

Sales Invoice Sequence Violation

saveInvoice was called before saveSales. The VSCU requires a strict two-step sequence.

Likely Causes

Developer followed OSCU documentation which uses a single combined call
Architecture migrated from OSCU without updating the sales flow
saveInvoice called in retry logic without first re-sending saveSales

Fix

VSCU requires two sequential calls in this exact order: (1) POST /trnsSales/saveSales with transaction metadata, then (2) POST /trnsSales/saveInvoice with invoice detail and receipt signature. OSCU combines both into a single POST /saveTrnsSalesOsdc call. These paths cannot be mixed.

Gotcha

Every developer building from OSCU documentation or an OSCU-first architecture hits this. There is no mention of this difference in any introductory guide.

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