BOLT 12: Allow sending amounts in invoice_request even if offer has it.

The actual amount can be complicated (x quantity, and proportional for
some recurring offers), but allow the sender to specify it, for
tipping purposes.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

12-offer-encoding.md

@@ -550,7 +550,9 @@ The writer of an invoice_request:
  - MUST specify `amount`.`msat` in multiples of the minimum lightning-payable unit
  (e.g. milli-satoshis for bitcoin) for the first `chains` entry.
  - otherwise:
- - MUST NOT set `amount`
+ - MAY omit `amount`.
+ - if it sets `amount`:
+  - MUST specify `amount`.`msat` as greater or equal to amount expected by the offer.
  - if the offer contained `recurrence`:
  - for the initial request:
  - MUST use a unique `payer_key`.
@@ -596,7 +598,6 @@ The reader of an invoice_request:
  - otherwise:
  - MUST fail the request if there is a `quantity` field.
  - if the offer included `amount`:
- - MUST fail the request if it contains `amount`.
  - MUST calculate the *base invoice amount* using the offer `amount`:
  - if offer `currency` is not the invoice currency, convert to the
  invoice currency.
@@ -638,6 +639,10 @@ The reader of an invoice_request:
  - otherwise (the offer had no `recurrence`):
  - MUST fail the request if there is a `recurrence_counter` field.
  - MUST fail the request if there is a `recurrence_signature` field.
+ - if the offer and request both contain `amount`:
+ - MUST fail the request if its `amount` is less than the *base invoice amount*.
+ - MAY fail the request if its `amount` is much greater than the *base invoice amount*.
+ - SHOULD use the request's `amount` as the *base invoice amount*.
 
 ## Rationale
 
@@ -659,6 +664,12 @@ BIP-32 derivation path); a valid system might be for a node to maintain a base
 payer key, and encode a 128-bit tweak here. The payer_key would be derived by
 tweaking the base key with SHA256(payer_base_pubkey || tweak).
 
+Users can give a tip (or otherwise obscure amounts sent) by specifying
+an `amount` in their invoice request, even though the offer specifies
+an `amount`. Obviously this will only be accepted by the recipient if
+the invoice request amount exceeds the amount it's expecting.
+
+
 # Invoices
 
 Invoices are a request for payment, and when the payment is made they