Mersal Invoicing & Serial Books
The problem
Invoice numbering looks like an administrative detail until an auditor asks why number 47 is missing, or why a proforma and a fiscal invoice share a sequence. Numbering is where billing either holds together or quietly stops being defensible.
How Mersal solves it
Mersal issues from three separate serial books — VAT, exempt and proforma — each with its own unbroken sequence. VAT is ruled per line, not per document, so a single invoice can mix rated and exempt work honestly. Once issued, an invoice freezes: corrections happen by auditable reissue, and the original stays on the record.
How it works
Three books, three sequences
A proforma is an estimate and never a receivable. It carries its own serial, is excluded from every revenue figure, and cannot be paid. Converting one to a fiscal invoice recomputes VAT from the stored per-line rates and stamps the charged tax, under a compare-and-swap so a double conversion cannot happen.
Frozen at issue
An issued invoice is a fiscal record of what was billed, so its stored values always win over current firm settings. Changing the firm's VAT percentage or currency later does not silently rewrite what a client was charged.
Delivery is evidence too
When a client first opens an invoice is recorded server-side, along with the acknowledgment if they give one. "I never received it" becomes a question with an answer.
What it prevents
- Gaps or collisions in a fiscal sequence
- A proforma inflating revenue or outstanding balances
- A settings change rewriting what a client was already billed
The fastest way to judge this is to watch it run against your own way of working.