Getting an invoice out and settled

The situation

A month of work is ready to bill. Some of it is VAT-rated, some exempt, and the client has asked for an estimate first.

What goes wrong without a system

The estimate and the invoice share a numbering sequence, VAT is applied to the whole document because that is what the template does, and when the client says they never received it there is nothing to point at.

How it runs in Mersal

1. Issue a proforma first

   The estimate issues on the proforma book, with its own serial. It is not a receivable and never counts toward revenue or outstanding.

2. Convert when the client agrees

   Converting recomputes VAT from the stored per-line rates and stamps the charged tax. The original terms carry over, and a double conversion is impossible.

3. Rule VAT per line

   Rated and exempt work sit on one document, each line ruled correctly, rather than forcing two invoices or one wrong one.

4. Send, and know it arrived

   The invoice is emailed with its PDF. When the client first opens it, that is recorded — as is their acknowledgment, if they give one.

5. Record the payment once

   Settlement records full or partial payment, guarded by an idempotency key so a retry cannot double-record it.

6. Issue the receipt

   A receipt issues on its own serial, sealed against later amendment, and goes to the client.

What it prevents

• An estimate inflating revenue
• VAT applied to a whole document because the template did
• "I never received that invoice"
• The same payment recorded twice