Updates the payment's own fields (amount, date, payment method, reference).

The invoice link is immutable — invoice_id in the body is ignored. full is only accepted when creating a payment — sending it here answers 422 with code parameter_not_allowed. A payment already declared to a tax authority cannot be updated — the request answers 422 with the lock reason.

An amount change re-evaluates the invoice: when the accumulated payments now cover its total, the invoice automatically transitions to paid, whatever its current state. Lowering an amount never reverts a paid invoice.