Why can I not edit a sent proposal or invoice?

The short answer

Because sent commercial documents are treated as formal history, not just draft notes. Sending locks the document so the client always sees exactly what you approved.

Proposals

Sent proposals do not return to draft. If the client needs changes, create a new proposal version and send that. The earlier version stays in the history as superseded, so nothing is lost.

Invoices

Invoices are editable while they are still drafts. Once sent, the document represents billing history and payment expectations, so it locks. If a sent invoice is genuinely wrong, void it and issue a corrected one rather than trying to change it in place.

What to do instead

For proposals: create a new version from the event’s Proposal view.
For invoices: void and re-issue, or create the next billing action.

If you work around document immutability incorrectly, approval and payment history become much harder to trust later.

Revise a sent proposal
Create an invoice from an accepted proposal
Proposals and invoices