Why a paid + completed invoice locks

A paid invoice locks. Once an invoice reaches Paid, GM stops accepting edits or status changes, and it cannot be cancelled. This is deliberate: a paid invoice is a financial record, and silently editing it after the money changed hands is exactly what you don’t want.

What locking means

Line items, amounts, and status are frozen on a paid invoice.
The status-change buttons disappear and a green “Invoice paid — no further changes allowed” notice shows instead.
Re-running a payment confirmation on an already-paid invoice does nothing and never resends the receipt.

Marking a completed service paid won’t un-pay it.Updating the service progress on a job that’s already paid used to be able to knock the invoice back to issued — that’s fixed. A paid invoice stays paid regardless of what happens to the service status afterwards.

Cancelling a paid invoice

A paid invoice cannot be cancelled. The money was collected and GM does not record refunds yet, so a cancelled label would hide a payment the books still hold. The service progress form does not offer Cancelled on a paid invoice. If you are refunding the customer, settle it with them directly.

Cancellation of an unpaid invoice is terminal too. See Mark an invoice as paid for how invoices get to Paid in the first place.